Several Paris rail stations were evacuated on Thursday after the CIA warned of a possible bomb attack.
The alert was lifted after police found nothing suspicious
Police said the CIA had told the French authorities of an anonymous message they had received warning of a possible attack between 1830 and 1930 GMT.
Traffic on the suburban RER line was halted and a number of stations were cleared in response to the tip-off.
The alert was lifted around 1930 GMT after police searches on the affected rail line found nothing suspicious.
"Following systematic searches of stations on RER A line, the police prefecture has requested the RATP (Paris region transport authority) to resume normal traffic," a police spokesman said.
A spokeswoman for the Interior Ministry told Reuters the alert had followed a tip-off from a Spanish CIA agent to the French DST counter-intelligence service of a possible attack on the Paris urban rail network.
She gave no more details on the nature of the threat indicated by the CIA.
France Info radio reported 40,000 to 50,000 passengers were stranded during the security alert.
France has been on high alert since the 11 March train bombings in Madrid, which killed 191 people.
A number of threats have been made to French interests in recent weeks.
Last month a railway worker found a bomb buried on the main train line between Paris and Switzerland.
French authorities have also received threats in letters from a self-professed Islamic group, whose authenticity is questioned.