The memo behind the claim that New York's subway system faces an imminent and credible threat of a terrorist attack has surfaced.

A joint Department of Homeland Security-FBI memo obtained by news agencies on Saturday said the attack is scheduled to take place on or around Sunday, with perpetrators using timed or remote-controlled explosives concealed in briefcases, suitcases or baby strollers.

The memo, issued Wednesday to state and local officials, said the department received information indicating the attack might be carried out by "a team of terrorist operatives, some of whom may travel or who may be in the New York City area."

The memo indicated homeland security and FBI agents doubted the reliability of the information; however, it also provided four pages of counsel on averting a possible attack.

On Thursday, U.S. forces in Iraq arrested two suspects believed to be involved in the alleged plot. A third suspect was reportedly arrested Friday, also in Iraq. It is believed those arrested received explosives training in Afghanistan.

Authorities are investigating whether a fourth had travelled to New York as part of the scheme, the Associated Press quoted a law enforcement official familiar with the case as saying.

The official, who spoke on condition of anonymity, told the Associated Press that a previously reliable informant described the man's trip to New York.

According to Department of Homeland Security spokesman Brian Doyle, however, the government has no information that the fourth suspect "is either here or even exists."

Officials believe the suspects had planned to travel through Syria to New York to meet with operatives and carry out the bombings.

In Baghdad, spokespeople for the U.S. military and the U.S. Embassy declined to comment about the arrests.

The entire New York City subway system has been on alert since Thursday, when officials warned of a specific threat of an attack on the transit network.

A former senior FBI official told CTV News that security officials in Washington did not think there was a credible threat and questioned the mayor's decision to warn the public.

"If I'm going to make a mistake, you can rest assured it is on the side of being cautious," New York Mayor Michael Bloomberg said in response to the criticism.

A U.S. counterterrorism official told Reuters that authorities were forced to act because the intelligence was specific, the threat came within a few months of the July 7 transit bombings in London and involved a U.S. city that has previously been attacked by Islamist militants.

New York has been on orange alert, or the second-highest level -- indicating a high risk of terror attack -- since authorities established the colour-coded warning system after the September 11, 2001 attacks.