Elon Musk’s underground Loop in Las Vegas suffered its first security breach just two weeks after opening this June, emails have revealed.

Information uncovered by tech industry website TechCrunch showed that an unauthorized non-Tesla vehicle joined the Loop system’s fleet of underground taxis on June 21, the final day of the International Beauty Show, presumably as a prank.

No one was hurt in the incident, which was handled peacefully by the Las Vegas Metro Police, the emails say. But the revelation is an embarrassment for The Boring Company (TBC), the firm behind the Loop, which had promised that physical barriers including gates and bollards between the Loop route and public roads would prevent this kind of situation.

Other information uncovered by TechCrunch includes TBC’s plan to increase the number of Tesla vehicles in the Las Vegas Convention Centre (LVCC) Loop from 62 to 70, and to allow the use of the cars’ autonomous technologies.

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So far, the TBC cars have all been operated by human drivers and with driver assistance technologies deactivated, contradicting what TBC had initially proposed to the LVCC when it pitched the Loop.

As TechCrunch points out, TBC had promised that “Tesla Autonomous Electric Vehicles (AEVs) will carry passengers in express, underground tunnels to three underground stations.”

But in emails between TBC president Steve Davis and Clark County Department of Building & Fire Prevention, the TBC boss admits that even if the Teslas did begin using their driver assistance technologies in the Loop, they still wouldn’t be autonomous.

“These are not ‘autonomous’ nor ‘self-driving’ vehicles,” Davis wrote in an email seen by TechCrunch that clarified the abilities of Tesla’s Autopilot system. Autopilot is currently being investigated by U.S. safety regulators after a serious of crashes.

But even if the Tesla could be made to operate in the loop autonomously, it seems that they’d be obligated to have a driver behind the wheel anyway to assist passengers in the event of an emergency.

Another legal requirement may mean June’s intruder vehicle isn’t the last non-Tesla to make an appearance in the Loop’s tunnels. The Loop must comply with the Americans with Disabilities Act (ADA), and as Tesla does not build a vehicle suitable for carrying disabled user, the TBC has been forced to buy a wheelchair-friendly low-speed electric vehicle TechCrunch believes could be a Tropos Motors Able.