WASHINGTON, Sept 19 (Reuters) - The assault on the U.S. consulate in Benghazi last week in which four Americans died was a "terrorist attack" that may have had an al Qaeda connection, a top U.S. counterterrorism official told Congress on Wednesday.

Rocket-propelled grenades and mortars struck the consulate on Sept. 11, the anniversary of the 2001 attacks on the United States. U.S. Ambassador Christopher Stevens and three other Americans died.

"They were killed in the course of a terrorist attack on our embassy," Matthew Olsen, director of the National Counterterrorism Center, said in response to a question at a Senate hearing.

Olsen said whether the attack was planned for Sept. 11 was under investigation, but the information so far indicated it was "an opportunistic attack" that "began and evolved, and escalated over several hours."

There were well-armed militants in the area, he said. "What we don't have at this point is specific intelligence that there was a significant advance planning or coordination for this attack."

Whether or not the attack was planned well in advance has become a point of dispute between the Obama administration and Republican lawmakers who say it bears the hallmarks of a premeditated assault. Senior Libyan officials have said the attack was planned in advance.

At the same hearing, Republican Senator Susan Collins said she agreed with Libyan officials that the attack was premeditated, planned and associated with the Sept. 11 anniversary. She expressed concern about the security at the consulate, where no Marines were present and security was handled by foreign nationals.

Olsen told lawmakers U.S. authorities are investigating who was responsible for the attack, and it appeared that a "number of different elements" were involved, including individuals connected to militant groups.

"As well, we are looking at indications that individuals involved in the attack may have had connections to al Qaeda or al Qaeda affiliates, particularly Al Qaeda in the Islamic Maghreb," he said.

"The picture that is emerging is one where a number of different individuals were involved, so it's not necessarily an either-or proposition," Olsen said. (Reporting by Susan Cornwell, Tabassum Zakaria and Donna Smith; Editing by Warren Strobel)

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