An Army official apologized Friday for conducting an unannounced training mission around the small city of Port Angeles, Wash., using special operations helicopters that the mayor said "terrorized" his city.

Dozens of alarmed residents called police to ask what was going on and said the noise and light from the mystery helicopters buzzing around the city panicked horses and other livestock, The Peninsula Daily News reported.

The Army said the helicopters involved included both twin-engine Chinooks and Blackhawk attack helicopters.

"No one had any warning about the helicopters, no one said anything afterward, and today city officials had to spend hours just trying to find out what had happened -- who had invaded Port Angeles," said Cherie Kidd, mayor of the Olympic Peninsula city about 60 miles west of Seattle.

The training exercise involved part of the 160th Special Operations Aviation Regiment, which is based at Fort Campbell, Ky., but has individual units in various locations, said Sgt. Jimmy Norris, an I Corps spokesman at Joint Base Lewis-McChord near Tacoma, Wash. Part of the 160th is based at Lewis-McChord, he said.

"Our watch commander last night reported that we received `dozens of calls' complaining about low-flying helicopters over the city," Deputy Police Chief Brian Smith said.

After multiple calls to Puget Sound-area military bases, Clallam County Sheriff Bill Benedict was finally able to determine about mid-day Friday that the helicopters belonged to the Army, the newspaper reported.

"Because of the volume of the complaints that we heard, I want to let the Army base know that if it's necessary to fly over populated areas, we want advance notice," Benedict told The Peninsula Daily News.

Army Col. H. Charles "Chuck" Hodges Jr., garrison commander at Lewis-McChord, told the newspaper that he had launched an investigation and was meeting with unit commanders at the base.

"I apologize, this is totally unacceptable," he said.

Hodges said the helicopters -- he mentioned four Chinooks -- were over Port Angeles from about 11:15 p.m. to shortly before midnight Thursday.

An Army spokesman for Hodges' office did not immediately return an Associated Press phone call Friday evening.

Kidd said she plans to meet Monday with Hodges at the base about 90 miles south of Port Angeles.

The helicopters were "training to work in urban environments," Norris told the AP, adding that the exercise involved landing at the small Port Angeles Coast Guard base. It's located across the city harbor from the downtown business area.

"I guess a lot of people got spooked because they had their lights on and the lights were bright, but I guess they had to have the lights on as part of the operation for landing at the airfield," he added.

The Daily News reported that the unannounced training mission prompted an immediate response on social media websites as hundreds of messages about the incident flooded Facebook and Twitter.

Port Angeles resident Eric Phillips said he saw four helicopters traveling in two sets, with the leading set flying without lights. The aircraft circled the city for at least an hour, he told Daily News.

Army Maj. Michael Burns, interviewed by telephone from Fort Campbell, told the newspaper the exercise also included Blackhawk helicopters.

He said he couldn't discuss many details of the mission or say why the Port Angeles area was chosen, only that "the particular area just gave a different training environment for the pilots, something unfamiliar.

"We do our best to try to avoid populated areas, but (with) those aircraft being so large and so loud, even if we're not very low, it seems very loud," Burns said.

Specific law enforcement agencies in the area were not notified of Thursday's training mission, Burns confirmed, saying that's not typically done for operations as short as a one-night exercise.

Also, the aircraft used in such missions can cover such a wide area, "it's tough to notify every agency," he told the newspaper.

Click here for more from The Peninsula Daily News.

The Associated Press contributed to this report.