Sen. John McCain said Sunday that China may be poring over a seized underwater drone to unearth secret information about Navy technology, hours after President-elect Donald Trump suggested Beijing should “keep it.”

“The Chinese are able to do a thing called reverse-engineering, where they are able to — while they hold this drone — able to find out all of the technical information. And some of it is pretty valuable,” McCain, R-Ariz., said Sunday on CNN’s “State of the Union.”

The comments by McCain, who leads the Senate Armed Services Committee, underscored the U.S. political tensions touched off by China’s decision to scoop up the submersible in international waters in the South China Sea. Assurances from China that the vessel would be returned failed to quiet U.S. critics — including Trump, who initially denounced the snatch-and-grab move and then reversed himself hours later.

Trump said on Twitter late Saturday that, “We should tell China that we don’t want the drone they stole back — let them keep it!”

McCain said China’s seizure was “a gross violation of international law,” echoing the U.S. response to the move and Trump’s initial blast via a tweet. The president-elect told his 17.5 million Twitter followers: “China steals United States Navy research drone in international waters — rips it out of water and takes it to China in unprecedented act.”

That broadside hit hours after the Chinese government said it had been in touch with the U.S. military about the Dec. 15 incident. The Pentagon said that China will return the vehicle after “direct engagement” between Washington and Beijing.

China’s ministry of defense pledged an “appropriate” return of the drone on its Weibo social media account, while also criticizing the U.S. for hyping the incident into a diplomatic row. It followed assurances from Beijing that the governments were working to resolve the spat.

The drone incident was disclosed by the Pentagon on Friday. China’s ministry said the U.S. “hyped the case in public,” which it said wasn’t helpful.

The Defense Department said a Chinese naval ship unlawfully seized the small unmanned vehicle Thursday while the USNS Bowditch, a U.S. Navy survey ship, picked up the drone in a routine operation 50 nautical miles northwest of Subic Bay in the Philippines.

The Chinese vessel was about 500 yards from the Bowditch when it launched a small vessel to retrieve the drone.

The tensions unleashed underscored the delicate state of relations between the two countries, weeks before Trump’s inauguration. Trump has threatened higher tariffs on Chinese products and questioned the U.S. approach to Taiwan, which Beijing considers part of its territory.

China restrains itself

For all the noise from Trump’s Twitter account and elements of the Chinese press, Beijing appears to be holding its fire at least until after he takes office. Until then, it looks set to continue the stance of “strategic composure” articulated after Trump questioned the U.S. policy of diplomatically recognizing Beijing instead of Taiwan.

Beijing will “strike back firmly” if Trump as president openly challenges China’s core interests like Taiwan, Tibet, the South China Sea and the East China Sea, said Shi Yinhong, director of the Center on American Studies at Renmin University in Beijing and an adviser to China’s State Council. Options include recalling the ambassador, stopping international cooperation, fighting a trade war, and even severing diplomatic relations.

“So far, China has adopted a cautious, measured approach of wait and see,” Shi said. “The government is still closely observing what Trump is up to and in the process of forming a clear view on his possible policy. This approach will likely continue into his presidency for the first couple of months.”

The fallout from worsening relations threatens a two-way trade relationship that reached $627 billion in 2015, more than U.S.’s combined commerce with Japan, Britain. and Germany.