U.S. officials timed the release of a $400 million payment to Iran until several American prisoners were released from the country, sources told Wall Street Journal Wednesday evening.

Federal officials refused to hand over the foreign currency until a Swiss Air Force plane carrying three of the freed Americans departed Tehran Jan. 17, according to the sources, a revelation that is sure to embolden critics who say that the payment was ransom money.

"It's time for the Obama White House to drop the charade and admit it paid a $400 million ransom to the world's leading state sponsor of terrorism," Republican National Committee Chairman Reince Priebus said in a statement Wednesday evening. "President Obama has foolishly put a price on the head of every American abroad and it should be no surprise that Iran has since detained more U.S. citizens. Hillary Clinton's support for this dangerous blunder shows once again she does not have the judgement to be president."

The administration says the U.S. owed Iran the money from a failed arms deal from the 1970s. The payment was the first installment of a $1.7 billion settlement that the administration announced it had reached with Iran in January.

The new discovery explains why an Iranian cargo plane was used to transport the money — euros, Swiss francs and other currencies — from Geneva to Tehran only after the Americans had been safely removed.