New York officials increased security after saying they were alerted to a credible, but uncorroborated terrorist threat.
But officials from the Department of Homeland Security are downplaying the threat and say it is of "doubtful credibility."
Despite the differing takes on the seriousness of the threat, New York officials mobilized more police officers.
Mayor Michael Bloomberg said the threat originated overseas, and was the most specific terrorist threat city officials had received to date. No one in New York has been arrested or detained, he said.
CBS News correspondent Randall Pinkston reports that while Bloomberg and Kelly said they would take the subway home tonight, a source tells CBS News that the matter may not be so trivial.
According to the source, the information came with specific details about how the attack would be mounted, how many people might be involved, and by what route the terrorists would approach the rail system.
CBS News correspondent Jim Stewart reports that the threat was picked up in chatter on the Internet.
"The people involved, approximately 19 or so, went into great detail how they might carry out this attack," Stewart said. "They talked about placing themselves as suicide bombers on the New York subway system using backpacks and briefcases — specifically using baby carriages."
But in Washington, Homeland Security Department spokesman Russ Knocke said "the intelligence community has concluded this information to be of doubtful credibility. We shared this information early on with state and local authorities in New York." Knocke did not elaborate.
A counterterror official, who was briefed about the threat by Homeland Security authorities, said the intelligence was considered doubtful because it did not reflect "on-the-ground, detailed" information. Rather, the official, who also insisted on anonymity, said the intelligence was similar to "what can be found on the Internet and a map of New York City."
Stewart reports that different branches of government are not regarding the threat with the same amount of gravity.
"Tomorrow, you will hear how this threat is perceived differently by the F.B.I. and the Department of Homeland Security, the C.I.A., the New York City Police Department. They all see it through a different prism."
Sen. Charles Schumer, D-N.Y., said he had spoken with Washington authorities, and the threat did not have the highest level of credibility or corroboration.
"Nonetheless, in a post 9/11 world you cannot be too careful," he said.