Police shut part of New York's Penn Station

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NEW YORK (Reuters) - Part of New York's Pennsylvania Station was sealed off on Friday morning, closing the portion of the station where Amtrak trains operate, police said.

The precaution came one day after New York officials said they had intelligence that the city's subway system was under a credible and specific threat of possible attack.

A policeman on the scene said only, "We are checking something out."

Commuters could still enter the subway at the station.

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A discarded soda bottle filled with an unidentified green liquid was found at the station during morning rush hour, Amtrak officials said. The substance did not pose a threat to passengers and was removed for testing.

A majority of the station was reopened by about 10:45 a.m., more than an hour after it was closed.

Officials in New York revealed the threat Thursday, saying an FBI source warned that terrorists had plotted to bomb the subway in coming days. But Homeland Security officials in Washington downplayed the threat, saying it's of "doubtful credibility."

A half mile away at the Port Authority Bus Terminal on Friday, more officers were visible on the streets, and one lane of traffic on Ninth Avenue was reserved for emergency vehicles