The bomb-laden craft that attacked a Saudi frigate last month was unmanned and likely Iranian, the U.S. Navy has determined.

Previously described as a Houthi suicide boat, the U.S. Navy has now determined the craft was unmanned and guided into the Royal Saudi Navy Al Madinah (702) remotely on Jan. 30.

“Our assessment is that it was an unmanned, remote-controlled boat of some kind,” Vice Adm. Kevin Donegan, U.S. 5h Fleet Commander said in a report in Defense News on Sunday.

USNI News confirmed the details with Navy officials on Monday.

Video of the attack obtained by USNI News earlier this month showed a small boat approaching the rear port quarter of the frigate at high speeds in a straight line before detonating at the waterline below the flight deck – apparently unchallenged by the ship’s company.

The explosion killed two Saudi sailors.

The unmanned boat was likely provided to the Houthi rebels by Iranian Revolutionary Guard Corps forces. The U.S. and other countries have intercepted arms shipments from Iran thought to be bound for the Houthi rebels. Houthi’s have used guided anti-ship cruise missile that are also used by Iran.

The IRGC, responsible for the costal defense of Iran in the Persian Gulf, has developed a variety of small attack craft based on commercial boats.

Those developments have included at least one unmanned surface craft – the Ya Mahdi.

Alleged to be based on a high-speed British hull, the unmanned fast boat has been in the Iranian inventory since 2010, according to reports.

According to a 2015 U.S. Army assessment on threats from unmanned craft, “utilizing suicide drones is an asymmetric strategy which both allows Iran to compete on an uneven playing field and poses a risk by allowing operators to pick and choose targets of opportunity.”

The asymmetric advantage the Iranians have provided the Houthi’s have been instrumental in extending the conflict, Afshon Ostovar, a professor at the Naval Post Graduate School who studies the IRGC, told USNI News earlier this month.

“The Houthis have managed to hold back the Saudi coalition forces since their early successes, which might be in part attributable to Iran’s continued support. The current stalemate is probably at least in part due to Iran’s enduring support to the Houthis,” he said.

The best-known small boat suicide attack was on guided missile destroyer USS Cole (DDG-67) which killed 14 sailors in 2000.

However, the unmanned attack boat makes it easier to put high dollar assets at risk, Donegan told Defense News.

“There are many others that don’t want to martyr themselves in making attacks like that and that’s pretty much where the Houthis are,” he said.

“So it makes that kind of weaponry, which would normally take someone suicidal to use, now able to be used by someone who’s not going to martyr themselves.”