WASHINGTON — American warships will prowl the waters off the Libyan coast and surveillance drones will buzz the skies overhead, but Defense Department officials said Thursday that catching the people who attacked the American consulate in Benghazi and killed the U.S. ambassador and three other Americans will involve an on-the-scene investigation led likely by Libyans.

A U.S. antiterrorism security team composed of about 50 Marines arrived in Tripoli, the Libyan capital, Wednesday and will be responsible for the safety of the embassy but would not be an active part of the investigation, officials said.

Pentagon spokesman George Little said that Libyan security officials know the region and are better equipped to respond more quickly at this time than U.S. investigators.

“We are ready to support, if called upon,” Little said. “It’s very early on in this investigation. There are no timelines, no details, yet. The Libyans are working closely with us. We’re grateful for their support.”

U.S. officials praised Libyan security and police forces, who on Thursday announced the arrest of four suspects in the attack.

But the FBI will play a role as well. Attending a conference in Doha, Qatar, U.S. Attorney General Eric Holder said that the bureau has opened an investigation into the deaths of Ambassador Christopher Stevens and the three other Americans who were killed during the attack on the consulate.

News reports indicated that the FBI was in Germany interviewing the 25 to 30 Americans evacuated from the Benghazi consulate.

Peter D. Feaver, a professor of political science at Duke University, said the practical value of a display of force like warships and drones might be minimal, but it adds significant symbolic value.

“By showing that we do have a presence, we’re making a statement, to people, to our enemies and to other governments in the region,” he said. “It’s important not to underestimate that message.”