NEW YORK (AP) -- Federal auto safety regulators are investigating why Tesla has repeatedly broken rules requiring it to quickly tell them about crashes involving its self-driving technology, a potentially significant development given the company's plans to put hundreds of thousands of driverless cars on U.S. roads over the next year.

The National Highway Traffic Safety Administration said in a filing on Thursday that Tesla's reports on "numerous" incidents involving its driver assistance and self-driving features were submitted far too late -- several months after the crashes instead of within five days as required.

The probe comes two months after the electric vehicle maker run by Elon Musk started a self-driving taxi service in Austin, Texas, with hopes of soon offering it nationwide. The company also hopes to send over-the-air software updates to millions of Teslas already on the road that will allow them to drive themselves.

Investors enthusiastic about such plans have kept Tesla stock aloft despite plunging sales and profits due to boycotts over Musk's support for U.S. President Donald Trump and far-right politicians in Europe.

The safety agency said the probe will focus on why Tesla took so long to report the crashes, whether the reports included all the necessary data and details and if there are crashes that the agency still doesn't know about.

Tesla did not respond to a request for comment, but the agency noted that the company has told it the delays were "due to an issue with Tesla's data collection," which Tesla says has now been fixed.

The new investigation follows another probe that began in October into potential problems with Tesla's self-driving technology in foggy weather and other low visibility conditions, which has led to several accidents including one death. That probe involves 2.5 million Tesla vehicles.

Tesla stock fell nearly 1% in midday trading Thursday to $321.