The White House apologised for keeping lawmakers in the dark regarding the exchange of an American soldier for five Taliban fighters, senators said on Tuesday, as controversy grew over the issue.
Administration officials plan a classified briefing for the full 100-member chamber on Wednesday, with lawmakers from both parties fuming over the trade, which saw captured soldier Bowe Bergdahl released on Saturday to US special forces in Afghanistan.
Senate Intelligence Committee chair Dianne Feinstein, a senior member of President Barack Obama's Democratic Party, said the White House breached US law when it failed to alert Congress to the proposed trade.
"It comes with some surprise and dismay that the transfers went ahead with no consultation, totally not following the law," she told reporters.
"We're very dismayed about it."
Feinstein reflected mounting bipartisan concern as lawmakers questioned the merits of releasing from Guantanamo five hardened Taliban fighters and officials in exchange for Bergdahl.
Seeking to mop up the political fallout, a senior White House official called senator Saxby Chambliss, the Intelligence Committee's top Republican, late on Monday "apologizing for not giving us advance warning," Chambliss said.
Feinstein said she too was called on Monday, by National Deputy Security Advisor Tony Blinken, who offered his apology.
Lawmakers have pounced on Obama for not giving Congress 30 days notice before releasing any detainee at the Guantanamo Bay military prison, as required in a law signed in December.
Chambliss and Feinstein pointed to such assurances made in 2012 by then secretary of state Hillary Clinton when a similar swap for Bergdahl was mulled.
Senate Democrat Carl Levin said Obama put Congress "on notice" last December in a signing statement saying he has constitutional authority to move quickly on detainees.
And senate majority leader Harry Reid said he was told of the swap on Friday and that he supported the exchange.
But few others openly backed the president. House speaker John Boehner said the only reason Obama did not give the necessary notice "is because the administration knew it faced serious and sober bipartisan concern and opposition."
Democratic senator Joe Manchin called the affair "very disturbing."
The swap will be debated in an open House hearing next week, with defense secretary Chuck Hagel invited to testify.
The controversy puts lawmakers in a tricky spot. Cognizant of the American military code to leave no soldier behind, they agree that securing Bergdahl's release after five years in Taliban captivity was admirable.
But lawmakers have blasted the swap for appearing to violate the principal of not negotiating with terrorists.
"I am concerned about what precedents we set here for exchanges, because I don't want the message to be, you can go ahead and capture Americans and use them to barter for others," senate foreign relations committee chairman Robert Menendez told reporters.
And lawmakers on Tuesday seized on administration rationale that the swap was rushed because Bergdahl's health had deteriorated sharply and his life was in immediate danger.
"I have heard no evidence that that is the case," said Senate Intelligence Committee member Susan Collins.
Meanwhile, CNN reported on Tuesday that a US Army fact-finding investigation conducted in the months after Bergdahl's 2009 disappearance concluded that he left his post deliberately, according to an official familiar with the probe who spoke with the network on condition of anonymity.
The official said there was no definitive finding Bergdahl had deserted because that would have required knowing his intent.
The probe also interviewed members of Bergdahl's unit, none of whom reported seeing him go, according to the official.