The attackers who killed 129 people in Friday night's wave of shootings and suicide bombings in Paris appeared to be made up of three teams, Paris Prosecutor Francois Molins said on Saturday, as a Greek official stated one of the gunmen traveled through Greece.
"We can say at this stage of the investigation there was probably three coordinated teams of terrorists behind this barbaric act," he told a news conference.
He also confirmed that French authorities had a security file for Islamist radicalization on one of the attackers, who also had a criminal record, but had never spent time in jail.
He added that a person who had hired one of the cars used in the attacks was stopped at the Belgian border.
In a statement issued on Saturday, the White House essentially backed France's position that ISIS was to blame for the attack.
"The team reviewed the intelligence picture, noting that we had no information to contradict the initial French assessment of ISIL's responsibility," the White House said in a statement after President Barack Obama met with his National Security Council.
It added that while there was no specific or credible threat to the United States, officials had reviewed their "homeland security posture."
A second man suspected to have been among the attackers in Paris on Friday is very likely to have entered Europe though Greece, Greek government sources told Reuters on Saturday.
Earlier, a Greek government minister said the holder of a Syrian passport found near the body of one of the gunmen who died in the attacks in Paris had passed through the Greek island of Leros in October.
"It is very likely that a second suspect also passed through Greece. The investigation is continuing," one of the sources said.