A chorus of Republican lawmakers condemned the Obama administration Wednesday for secretly sending Iran $400 million in what critics called a ransom payment.

The U.S. shipped pallets to Iran loaded with millions of dollars worth of foreign currency around the time that the Islamic Republic freed four American hostages, according to the Wall Street Journal.

The January payment was the first in a $1.7 billion legal settlement between the U.S. and Iran over a decades-old failed arms deal. Officials have not said when the money arrived or how it was paid for, despite numerous congressional inquiries.

House Foreign Affairs Committee Chairman Ed Royce, who sent a letter to Secretary of State John Kerry in February asking for details about the payment, denounced the administration's lack of transparency in a statement.

"The logistics of this payment – literally delivering a plane full of cash to evade U.S. law – shows yet again the extraordinary lengths the Obama administration will go to accommodate Iran, all while hiding the facts from Congress and the American people," Royce said.

The administration has rejected the notion that the payment was a quid pro quo, and said Wednesday that the simultaneous hostage exchange, cash shipment, and implementation of the Iran nuclear deal were "very separate efforts."

Royce expressed concern that Iran would use the funds for nefarious activities, and said that paying Iran for prisoners would endanger more Americans.

"Hundreds of millions in the pockets of a terrorist regime means a more dangerous region, period," he said. "And paying ransom only puts more American lives in jeopardy."

After the State Department sent Royce a partial response to his February letter, the California congressman wrote a follow-up for further details. The administration has yet to respond.

The State Department claimed Wednesday that it had been open with Congress.

"I can assure you that we don't do anything without notifying Congress, regardless of what that may be," said State Department spokesman Mark Toner. "We always make Congress aware of whatever actions we're taking."

Kansas congressman Mike Pompeo, who also sent a letter to the State Department in January and was met with an incomplete answer, compared the administration's behavior to that of the Iran-Contra scandal.

"Reports of the U.S. paying Iran $400 million in pallets of cash, using other currencies in order to circumvent American law, is something of Hollywood films—and is reminiscent of the Iran Contra scandal," Pompeo said. "Have we not learned we cannot trust the world's largest state sponsor of terrorism?"

He also denounced the administration's claim that the timing of the events was coincidental.

"To tell the American people that the payment of $1.7 billion on the same day as four American hostages were released was just coincidence insults our common sense and borders on lawlessness," Pompeo said.

Pompeo and Texas senator John Cornyn filed legislation in May that would force the administration to reveal how they made the payment, since Iran-U.S. dollar transactions are illegal, and whether it was for ransom.

Illinois congressman Peter Roskam, Arizona senator John McCain, Illinois senator Mark Kirk, Texas senator Ted Cruz, and a number of other lawmakers also condemned the "ransom payment" in statements Wednesday.