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.
The holder of a Syrian passport found near the body of one of the gunmen who died in the attacks passed through Greece in October, a Greek minister said, and another suspected attacker was thought to have entered Europe the same way.
"The holder of the passport passed through the island of Leros on Oct. 3, 2015, where he was identified according to EU rules," Greece's deputy minister in charge of police, Nikos Toskas, said in a statement.
Toskas did not know if the Syrian passport had been checked by other countries through which the holder might have passed on his way to France.
A Greek police source said the passport's owner was a young man who had arrived in Leros on a small vessel from Turkey with a group of 69 refugees and had his fingerprints taken by Greek officials.
Three Greek government sources later said a second suspect was also very likely to have come into Europe through Greece, adding that an investigation was still under way.
Separately, Belgium's federal prosecutor's office says authorities have so far made three arrests linked to the deadly attacks in Paris.
Spokesman Jean-Pascal Thoreau says the arrests at the Belgian border came after a car with Belgian license plates was seen close to the Bataclan theater in Paris on Friday night, one of the places where victims were killed.
He said it was a rental vehicle and police organized several raids in the St. Jans Molenbeek neighborhood in Brussels on Saturday.
Elsewhere, Muslim leaders in the United States are condemning the attacks In Paris and offering condolences and prayers for the people of France. Nihad Awad, the executive director of the Council on American-Islamic Relations said "we are revolted by this heinous and despicable attack on civilian populations."
He says the Islamic State group, which claimed responsibility for the attacks that killed at least 129, "does not represent Muslims."
Oussama Jammal of the U.S. Council of Muslim Organizations called on the American Muslim community to hold candlelight vigils to remember the victims.
Imam Johari Abdul-Malik of the Muslim Alliance in North America called on Muslims to "redouble our efforts in vigilance and confronting extremism."
--The Associated Press contributed to this article.