Subways threatened?

CNN reports that the New York subways are being threatened and that terror alert levels there will rise.

NEW YORK (CNN) -- The FBI has shared a "specific threat" with New York officials against its subway system, Mayor Michael Bloomberg said Thursday.

"If you see something, say something. Call ... 911 if it's an emergency," he said, urging vigilance.

Bloomberg added that he plans to travel on the subway Thursday and Friday.

There were indications that a terrorist attack on the subway system was possible "in coming days," said New York Police Commissioner Raymond Kelly.

The subway system carries up to 7 million commuters daily.

The heightened alert came 10 days after the arrest of nine people in France allegedly plotting attacks on the Paris subway, an airport and an intelligence agency's headquarters.

"We can't be too careful, and the precautions that New York City has taken are appropriate and make a good deal of sense," said Sen. Charles Schumer of New York, a Democrat. "We cannot be in a situation where we don't turn over every stone. But the bottom line is that in the post-911 world, you can't be too careful."