Portland police have joined the Coast Guard investigation into how two large commercial schooners at the Maine State Pier became adrift overnight, and one of the boat owners says she is certain it was an act of vandalism.

The Coast Guard received a call at 4:30 a.m. from a worker on the waterfront who reported that the 88-foot Wendameen was adrift and pinned against a work barge at the pier. The Coast Guard dispatched a small boat to check on it and when it arrived, the crew realized another schooner, the 74-foot Frances, was not at its customary berth.

The Frances was discovered adrift and up against a sport fishing boat at Sunset Marina on the opposite side of Portland Harbor, said Lt. Scott McCann of Coast Guard Station South Portland.

Michelle Thresher of the Portland Schooner Co., which owns the Wendameen, said it’s obvious somebody set both sailboats adrift. The Frances is operated by Maine Sailing Adventures. The boats carry passengers on day cruises on Casco Bay.

Each boat was tied to the pier with eight lines, and all the lines were still on the boats and in perfect condition when the Coast Guard found them, Thresher said.

“This was somebody doing something malicious,” she said.

She said she has not seen the security video, but believes the boats were set adrift around 4 a.m. She said she and Megan Jones, an owner of the Frances, have filed incident reports with police.

Both sailboats appeared to have sustained only cosmetic damage. The crews of the two boats had doubled up their lines Wednesday night to secure the vessels against heavy weather, McCann said. The astronomical high tide of more than 11 feet and winds gusting to at least 23 knots may have played a role, he said, but investigators also are looking at security footage at the pier to determine whether the vessels were intentionally set adrift.

“We’re trying to cover all our bases,” McCann said. “Boats shouldn’t go adrift in the middle of the night.”

Police Sgt. Dean Goodale said none of the lines on either boat were damaged, raising the possibility that someone intentionally untied them.

Staff Writer Tom Bell contributed to this report.