Federal authorities announced Wednesday word of a possible terror plot against the city subway and train systems during the holiday season, which has prompted increased security on the Long Island Rail Road and other systems.

An internal memo obtained by the Associated Press says the FBI has received a "plausible but unsubstantiated" report that al-Qaida terrorists in late September may have discussed attacking the subway system.

A U.S. counter terror official told the Associated Press that senior government officials have been briefed because the FBI very recently received credible information about possible attacks over the holiday season, and authorities are particularly concerned about this long holiday weekend.

Some riders say they?re taking every threat that comes along seriously, while others shrug it off.

"I mean seems to me there's so much security around and to me [it] seems no more dangerous than another day on the street,? Joe Urban, visiting Long Island from Buffalo, says.

Rep. Peter King (R-Seaford), the top Republican on the House Homeland Security Committee, says authorities "have very real specifics as to who it is and where the conversation took place and who conducted it."

"It certainly involves suicide bombing attacks on the mass transit system in and around New York and it's plausible, but there's no evidence yet that it's in the process of being carried out," King says.

The Metropolitan Transit Authority released the following: ?We are already on high alert and are always on a heightened state of readiness during this season, as always, we ask that our customers remain alert and report any suspicious activity or packages."

AP wire reports contributed to this story.