Sen. Angus King joined Democratic senators in demanding information about President Donald Trump’s intent to accept a $400 million airplane as a gift from the Qatari royal family, saying the gift is unconstitutional and threatens national security.

Maine’s Republican senator, Susan Collins, and Rep. Chellie Pingree, D-1st District, did not sign the letter but also criticized the idea of accepting the gift for security and ethical reasons.

Earlier this week, Trump said he would be “stupid” not to accept the free gift of a luxury plane.

But King and the Democratic senators said in a letter to Secretary of Defense Pete Hegseth and Secretary of the Air Force Troy Meink that the Boeing 747-8 aircraft offered to Trump as a gift could require $1 billion worth of investments to identify and address any security vulnerabilities before it could be used as Air Force One.

“(We) write today with alarm over the dangers to operational and national security presented by President Trump’s desire to execute an unconstitutional and unseemly acceptance of a $400 million gift from the Qatari royal family in the form of a luxury Boeing 747-8 aircraft,” they wrote in the May 14 letter. “We are especially concerned about the operational security and counterintelligence risks of potentially using this aircraft for sensitive Presidential travel — and the massive cost to American taxpayers to identify and close critical vulnerabilities.”

Trump, who was traveling in the Middle East this week with stops in Saudi Arabia, Qatar and the United Arab Emirates, is looking to put the jet into service before his term ends in 2029. If accepted, it would be one of the largest foreign gifts ever received by the U.S.

Trump also has raised ethical questions by saying the plane would eventually be donated to his presidential library, while maintaining he has no plans to use it for personal reasons after leaving office.

Republican U.S. Sen. Susan Collins said in a written statement that the gift is “rife with legal, ethical, and practical impediments, including the potential for espionage.”

“I’m not sure how we would be able to adequately inspect and outfit it to prevent that from happening,” Collins said. “Also, by the time the plane is done, the President’s term may well be nearly finished. Ultimately, I’m not sure why this is necessary at all.”

Trump has touted the deal as saving U.S. taxpayers money, but King and the 11 Democratic senators who signed the letter, including Minority Leader Chuck Schumer, said that’s not the case.

In addition to the significant investment needed to address security concerns, taxpayers are already paying $3.9 billion for two new aircraft being built by Boeing, they said.

Those planes are slated to be finished by 2027, three years behind schedule.

“This administration has been vocal about its commitment to efficiency and cost-savings in the federal government,” they said. “But spending taxpayer money on efforts to upgrade this Qatari jet – when the President currently travels securely – is unnecessary and wasteful.”

Earlier this week, King told Scripps News that the gift was likely aimed at gaining influence, calling it “crazy on every level” and that “it doesn’t pass the straight-face test.”

“Is it really a good idea to accept a gift this extravagant from another country? What are they getting in return? What are they buying? Are they doing it just because they’re really nice people? No,” King told Scripps.

The letter included a list of 10 questions and set a deadline of June 1 for answers.

“Air Force One is host to some of the most sensitive conversations in our government. This jet poses the risk of foreign surveillance and influence at the highest level of our government, to say nothing of the propriety of accepting it,” U.S. Rep. Jared Golden, D-2nd District, said in a statement. “I’ve sponsored legislation in Congress to prevent exactly this kind of undue foreign influence on our democratic system. The United States should reject this ‘gift.’”

U.S. Rep. Chellie Pingree, D-1st District, agreed that the plane would present a “massive national security threat.” She said in a written statement that the luxury jet donated by a country known for human rights abuses and terrorist ties “is just the latest example of the grift and corruption that have come to define the Trump Administration.”

“The President’s claim that the jet was ‘free’ and therefore no big deal is as laughable as it is delusional,” Pingree said. “That the jet comes from Qatar—a country infamous for its human rights abuses, and with known ties to several terrorist groups—makes this proposition even more outrageous. At a time when the Administration is alienating allies around the world, this is who they choose to cozy up with?”