SAN FRANCISCO — Officials said Tesla is not party to a probe into a weekend Model S crash that killed two — a departure from typical protocol and a potential sign of the auto company’s strained relations with investigators.

National Transportation Safety Board spokesman Keith Holloway said Tuesday was investigators’ first full day at the scene of the crash in a Houston suburb, and they were probing the vehicle’s operation and the subsequent fire. He added Tesla was “working with” investigators “but is not a party to the investigation.”

The NTSB booted Tesla as a party to its probe into another fatal crash in 2018 after the car manufacturer disclosed investigative information before the investigation’s conclusion.

Holloway referred questions on Tesla’s involvement to the company, which has disbanded its public relations department. The company did not respond to a request for comment.

On Monday, Elon Musk tweeted that Tesla data “so far” indicated the car’s Autopilot driver assistance system wasn’t in use during the crash. Autopilot is a Tesla driver assistance system that can navigate from highway on-ramp to off-ramp, can detect stop lights and stop signs, and can self-park and summon the vehicle. Police previously said there wasn’t anyone in the driver’s seat during the crash.

Two men died after a Tesla vehicle, which authorities said was operating without a driver, crashed into a tree in a Houston suburb on April 17. (Reuters)

According to Reuters, the Harris County Constable said it planned to serve search warrants to Tesla to secure data from the car that crashed following Musk’s tweet. The Harris County Constable Precinct 4 did not return repeated messages requesting comment.

Musk also tweeted Tuesday about the National Highway Traffic Safety Administration, which has taken a more hands off approach to safety regulation of the electric carmaker.

“NHTSA is great,” he wrote.

Investigators are probing how two men were killed Saturday when a Model S sedan veered off the road, leading to a fire that took hours to extinguish, according to police.

The NTSB said Monday it was sending a team of investigators to investigate the crash and subsequent fire, which smoldered for hours and required more than 30,000 gallons to put out.

“Our investigation has determined that one of the victims was in the front passenger seat; one was in the back seat,” Mark Herman, a constable for Harris County Precinct 4, told the station KHOU, adding that police were “100 percent certain” that the driver’s seat was unoccupied.

Tesla has said that drivers should pay full attention while using Autopilot.

In past investigations, officials have required Tesla’s assistance to decode data from Tesla’s proprietary onboard systems. That generally makes the automaker’s cooperation critical to federal safety investigations.

Tesla was removed from an investigation into the death of 38-year-old Apple engineer Walter Huang, who was killed when his Model X SUV slammed into a highway barrier in California in 2018. The NTSB ultimately cited Autopilot system limitations, the driver’s distraction from a cellphone game, and apparent overreliance on the Autopilot system, in the crash.

Before the investigation’s conclusion, Tesla issued a preliminary explanation in a blog post, where it said that Autopilot was activated moments before the crash and the car’s cruise control distance was set to a minimum, meaning it would tail somewhat closely behind other cars.

“At this time the NTSB needs the assistance of Tesla to decode the data the vehicle recorded. In each of our investigations involving a Tesla vehicle, Tesla has been extremely cooperative on assisting with the vehicle data. However, the NTSB is unhappy with the release of investigative information by Tesla,” NTSB spokesman Chris O’Neil said at the time.

The NTSB ultimately issued its findings in 2020, and went on to criticize federal regulators for a “lack of leadership” on technologies promising what it regarded as partial automation to drivers. The NTSB has also called directly for NHTSA to impose stricter standards on automation, most recently when chairman Robert Sumwalt issued a letter to the agency urging stricter standards earlier this year.

NHTSA did not respond to a request for comment on the Tesla probe on Tuesday. The agency said Monday it was aware of the “tragic crash” and launched a special investigation team to look into the matter. It was one of 28 investigations into Tesla vehicle crashes the agency has initiated, it said, including four completed cases.

In an NTSB investigation, party status is limited to “those persons, Federal, state, or local government agencies and organizations whose employees, functions, activities, or products were involved in the accident and that can provide suitable qualified technical personnel to actively assist in an investigation,” according to federal regulations published online by Cornell Law School.

The parties, the regulations say “must follow all directions and instructions from NTSB representatives.”

But the relationship is not guaranteed.

“Party status may be revoked or suspended if a party fails to comply with assigned duties and instructions, withholds information, or otherwise acts in a manner prejudicial or disruptive to an investigation,” it adds.