WASHINGTON—The U.S. ambassador to the United Nations has denied she tried to mislead Congress when she initially called the Sept. 11 attack on the American consulate in Benghazi, Libya a spontaneous protest.

Susan Rice, in a letter released Friday, wrote three Republican senators that her comments on television news shows after the attack were based on the best information available at the time from intelligence officials.

U.S. Ambassador Chris Stevens and three other Americans were killed. Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee, has said the State Department refused repeated requests for increased security at the consulate.

Rice wrote Sens. John McCain of Arizona, Kelly Ayotte of New Hampshire and Ron Johnson of Wisconsin that she "consistently qualified, in some instances twice, the information I provided as preliminary." U.S. officials modified their initial public assessment and said the attack was an act of terrorism, with some attackers linked to groups affiliated with, or sympathetic to, al-Qaida.

Rice said on NBC's "Meet the Press" Sept. 16 that the attack was "initially a spontaneous reaction" to a demonstration in Cairo. The Egypt demonstration was a protest against an anti-Muslim film made in the United States. However, Rice also said, "Obviously, that's our best judgment now. We'll await the results of the investigation...."

The U.S. intelligence community and the FBI are current investigating the attack.

The senators said in a joint statement that they did not accept Rice's explanation.

"The Obama administration failed to sufficiently protect our consulate and diplomats in Benghazi in the face of obvious and growing threats in eastern Libya in the months leading up to the September 11, 2012, terrorist attack," they said. "To make matters worse, the administration mishandled its response to the attack and appears to have selected intelligence that mischaracterized the attack and misled the American people. "

The senators contended that some intelligence officials, within hours of the attack, told administration officials that militants connected to al-Qaida were involved.

"Either the Obama administration is misleading Congress and the American people, or it is blaming the entire failure on the intelligence community," the senators said. They added the administration's conduct after the attack "indicates a breathtaking level of incompetence."