PARIS -- The French government is on alert for terrorist threats and "leaving nothing to chance," the interior minister said Friday, a day after dozens of train and Metro stations were evacuated on a CIA tip. The CIA contacted French authorities on Thursday, warning of an attack on Paris' commuter network, the RER, at the end of rush hour between 8:30 p.m. and 9:30 p.m. No attack materialized, but Interior Minister Dominique de Villepin said the information was from a credible source and had to be taken seriously.

"In a period where international threats exist," de Villepin said, "we must be on alert and respond together to signals that could be addressed to us."

"We are leaving nothing to chance," de Villepin told Europe-1 radio.

The warning came from the CIA's Spanish station, which intercepted an e-mail from Madrid that referred to an attack against a "red line" in Paris at a "central station," a police official close to the investigation said on condition of anonymity.

On the Paris Metro map, the RER-A line is designated in red. The line crosses Paris, linking the capital to its suburbs in the west and east. Authorities didn't have time to verify the threat but determined that the RER-A line could be at risk, the police official said.

At 8:15 p.m. Thursday, fifteen minutes before the designated warning time, authorities halted traffic on the RER-A line. Some 40,000 people usually use the line at that time of night.

"Faced with a situation of risk, faced with information given to us by the American services, it was essential for us to apply the principle of responsibility, the principle of precaution," de Villepin said. "The mobilization of our services is complete, as is our vigilance."

All stations on the line -- more than 40 in all -- were closed and passengers evacuated. Also evacuated were Metro stations that connect the subway network to the RER line, police and transport authorities said.

The alert was lifted just after 9:30 p.m. and traffic progressively resumed after the timeframe mentioned by the CIA had safely passed.

French authorities have been on high alert at airports, train stations and other sensitive sites since the March 11 bombings on four commuter trains in Madrid, Spain, that left 191 people dead and more than 1,800 injured.

On Friday, a train carrying 250 passengers heading from the eastern city of Strasbourg to Paris was evacuated and delayed for three hours after a suspect bag was found aboard, the rail authority said. The bag, examined by bomb experts, contained clothes and shoes, officials said later.