The White House had information that Islamic radicals were claiming credit for the attack on the U.S. Consulate in Benghazi, Libya, two hours after the assault began.
U.S. Ambassador to Libya Chris Stevens and three other Americans died in the Sept. 11 attack.
For more than two weeks the White House repeatedly blamed the incident on protests against an anti-Islamic movie that turned violent.
Now, Reuters news service reports it has obtained several emails sent from the State Department's Operations Center to the White House and the Pentagon as the attack was going on.
The first two emails describe the attack and efforts to locate the ambassador and his team.
The third email states in its subject line: "Ansar al-Sharia claims responsibility for Benghazi attack."
The message added that the militant group made the claim on Facebook and Twitter and called for attacks on the U.S. Embassy in Tripoli.
Reuters said it received the emails from an anonymous source not connected with the State Department or U.S. intelligence agencies.
The information raises new questions as to why the White House took so long to acknowledge the incident as a terrorist attack.