The regulators knew Musk could be impulsive and stubborn; they would need to show some spine to win his cooperation. So they waited. And in a subsequent call, “when tempers were a little bit cool,” the official said, Musk agreed to cooperate: “He was a changed person.”

Since that success in 2016, officials have learned to work with Musk, using a combination of pressure, flattery and threats to persuade him to comply with federal safety measures, according to a half-dozen former regulators, some of whom spoke on condition of anonymity to discuss sensitive matters. In the past six months, Tesla has issued at least a dozen voluntary recalls, a dramatic turnaround for a company known to quietly issue software updates direct to cars — without alerting the public — to fix sometimes alarming safety problems.

With about 2 million cars on the road, Tesla recently has experienced a wave of troubles: Cars using its driver-assistance features have slammed on the brakes for no reason and rolled through stop signs — the latter because Tesla programmed them to do so. A string of crashes into parked emergency vehicles is under investigation. And the cars’ batteries have been documented exploding in crashes and while parked in garages.

Such issues typically prompt NHTSA to investigate and sometimes to push for voluntary or mandatory recalls. If an automaker refuses to cooperate, NHTSA can impose cash fines of about $23,000 per day. The threat of fines — which can add up to nearly $115 million — generally works with traditional companies, the former officials said, but hasn’t proven effective when dealing with Tesla, an extraordinarily valuable company owned by the richest man in the world.

So NHTSA officials have turned to less conventional strategies to force the electric vehicle manufacturer to be more transparent about safety issues — a critical matter at a time when more than 50,000 drivers can now use Tesla’s “Full Self-Driving” software to navigate the nation’s public roads.

“Tesla is presumably smart enough to realize when they don’t have the upper hand anymore,” said Phil Koopman, an associate professor at Carnegie Mellon University whose focus includes federal auto regulations. “Tesla has a choice to make — they have to decide whether to cave or go to the mat. And the reality is, on [federal safety regulations] they’re going to lose.”

Tesla and Musk did not respond to specific questions in a detailed request for comment. Musk said in an email, “For the 100th time, please give my regards to your puppetmaster,” referring to Amazon founder Jeff Bezos, who owns The Washington Post. In a subsequent email, he also criticized The Post’s paywall for online articles.

NHTSA declined to make anyone available for an interview, citing ongoing investigations. But the agency issued written statements to The Post expressing its commitment to protect public safety.

Tesla has publicly touted the safety of its vehicles and its driver-assistance technology, boasting that it has won the highest possible score — five stars — in crash tests where NHTSA slams cars into barriers and then examines the results. Tesla also has said its cars have the lowest probability of injury of any car tested, though NHTSA has disputed that characterization.

Musk’s own attitude was part of the problem with efforts to enforce safety, the officials said. Some experienced personal encounters with Musk that escalated into yelling matches or otherwise proved unproductive because of the CEO’s skepticism about their findings.

NHTSA’s experiences with Tesla were unique among major automakers, the officials said. It was not rare for companies under scrutiny to fiercely push back against their findings, sometimes resulting in mandatory rather than voluntary recalls, they said. But with Tesla, issues as simple as a malfunctioning heat pump or noncompliant sound effects to alert pedestrians to a vehicles’ presence could result in stubbornness.

Tesla eventually issued recalls in both cases — decisions influenced by NHTSA, a spokeswoman said.

“NHTSA will ensure that vehicle manufacturers and developers prioritize safety while they usher in the latest technologies,” spokeswoman Lucia Sanchez said.

Regulators have been slow to take action on some software suites that power automated features, in part because they are wary of appearing to stifle emerging technologies, the former officials said. There also are few rules governing these technologies, further hindering efforts at regulation.

Since 2016, NHTSA has opened 31 special crash investigation cases involving advanced driver-assistance technology, according to data provided by the agency. Twenty-four have involved Tesla vehicles.

Safety experts and some of the former regulators who spoke with The Post raised concerns about “Full Self-Driving” in particular because of its experimental nature. Tesla says the software is in “beta,” meaning it is a pilot through which the company hopes to learn and improve its features for an eventual full release.

Lawmakers have pressed for more transparency regarding Tesla’s practices. In February, following a Post report on the cars’ sudden braking, Sens. Richard Blumenthal (D-Conn.) and Edward J. Markey (D-Mass.) criticized Tesla for putting software on the roads “without fully considering its risks and implications.” They urged NHTSA “to continue taking all appropriate action to protect all users of the road.” And they called on the Federal Trade Commission to launch an investigation into what they called “misleading advertising and marketing” of Autopilot and “Full Self-Driving” systems.

During much of the Obama administration, Tesla slid under the radar of federal safety regulators. As a niche automaker delivering at most tens of thousands of luxury cars per year, officials said it would not have ranked on the agency’s priority list, compared with high-volume automakers such as Ford and Toyota. From 2013 to 2015, there was just one recall per year for early Tesla Model S luxury sedans. Those recalls — which involved seat back mountings, charging equipment and incorrectly secured seat belts — compare with a total of 2,261 vehicle recalls over the same period, according to NHTSA data.

One early run-in set the tone for regulators’ interactions with Tesla. In 2013, Tesla claimed its Model S was the safest car ever tested by NHTSA.

Officials at the agency were dumbstruck, according to some of those who spoke with The Post. The agency issues up to 5-star ratings, but it does not take its designations beyond the star score.

At one point, two former officials recalled, NHTSA officials threatened to contact the FTC, which regulates marketing.“If Tesla wasn’t willing to pull the plug, FTC was probably going to take action,” one of the former officials said.

But Tesla continued to make similar claims, including in 2019, when it said its Model 3 had the “lowest probability of injury of any vehicle ever tested” by NHTSA. The agency ultimately referred the automaker to the FTC. The FTC declined to comment.

NHTSA, meanwhile, focused increasing attention on the automaker as it built more and more cars, launching the Model X SUV in 2015 and steadily growing its production in the buildup to the mass market-aimed Model 3. The agency also had to deal with emerging issues, such as fires caused by road debris striking the cars’ underbodies, where the high-voltage battery is located.Musk wrote in a statement at the time that Tesla would introduce a fix to bring that risk “down to virtually zero.”

One official recalled that regulators were confused by Musk’s reluctance to address one battery-related issue. The only evident solution was to appeal to his sense of pride.

“NHTSA staff backs Musk into a corner and challenges his ego and says, ‘Wait, you can’t solve this?’ ” a former official said. “And the next day he has a solution.”

The Trump administration was even less hands-on. The NHTSA website lists only one recall for the 2017 through 2020 Tesla Model 3, then the automaker’s most popular model. Depending on the model year, there were up to eight recalls issued for the same car beginning May 25, 2021, after President Biden took office.

Under the Trump administration, career officials and top staff were reluctant to take a strong stance on a mounting catalogue of safety concerns, wary of appearing to target innovation and already under pressure from the president to ease regulations.

The officials said staffers were reluctant to take on a rapidly emerging automotive power with a massive public platform, worried they wouldn’t have the backing of top officials if they received a browbeating from Musk.

“My former staff felt they had nothing to do — twiddling their thumbs,” one former official said. “The professional staff — the career staff — wanted to do stuff either from a regulatory standpoint or investigation perspective” but were stymied.

Musk praised NHTSA as “great” in April 2021.

Although Labor Secretary Marty Walsh recently met with Musk and took a factory tour in Texas, Axios reported, Musk has criticized President Biden for leaving Tesla out of major events. Musk also has parroted Republican barbs, joking at one point Biden was “sleeping.”

Last summer, NHTSA began requiring companies such as Tesla to report on certain crashes involving automated features within one day of learning of the incident. Then in August, the agency launched an investigation into about a dozen crashes involving parked emergency vehicles while Autopilot was active. In a letter, it called out Tesla for pushing out a software update to help its cars better see emergency vehicles, without formally issuing a recall.

A surge of investigations, recalls and public admonitions has followed.

Former officials who spoke with The Post said Tesla’s haphazard approach has grated on some NHTSA staff, and the enforcement reflects an attempt at a course correction.

“The agency does hold a very firm line on [federal motor vehicle regulations], and I don’t think any of us want to live in a world where the automakers do essential recalls without going through that process,” said Bryan Thomas, who was communications director at NHTSA during the Obama administration. “If the car’s going to react differently at a stop sign tomorrow than it did today, you should know that as a driver.”

The former officials said NHTSA is not singling out Tesla. Instead, they said the agency is using a calculated approach to try to force the combative automaker to recognize findings that can’t be denied. That means picking targets — the seat belt chime, rolling stops, a windshield defroster — that may strike executives and even some owners as trivial. But those narrow targets offer an entree to larger issues that are not yet addressed by federal regulations, which often lag behind the latest software advances.

In February, NHTSA cracked down on a feature known as “Boombox,” which plays sounds that bystanders can hear — such as an ice cream truck jingle — but can drown out sounds that warn pedestrians of approaching vehicles. On Twitter, Musk decried the agency as the “fun police.”

Federal safety investigators from the National Transportation Safety Board have also focused their attention on Tesla. Late last year, NTSB Chair Jennifer Homendy wrote a letter to Musk calling out the company’s “inaction” on recommendations that would have added safeguards to the Autopilot system in response to the fatal 2016 crash with the tractor-trailer.

In a statement posted to its website at the time, Tesla called the death “A Tragic Loss.” The company defended the performance of Autopilot across the tens of millions of miles it had logged and emphasized that drivers should be prepared to take control of the vehicle at any time.

In September, Homendy expressed her concern about Musk’s approach to safety in an interview with The Post.

“I think Elon Musk is an incredible innovator,” she said, expressing hope for “the ultimate success of [autonomous vehicle] technologies — which could save lives.”

But she also encouraged Musk to “really prioritize safety for his company,” adding: “I don’t want to see lives lost in the meantime.”