BELFAST, Maine — Police are calling it the mystery of the oily gulls.

For the past few days, grease-covered gulls have been appearing along the waterfront and in the yards of nearby residences. Five have been captured and turned over to a bird rehabilitation center, but sightings of others continue to be reported.

“We had complaints about sick birds and we found three that appear to be covered with oil,” Police Chief Jeffrey Trafton said Tuesday. “They can’t fly. You can walk right up to them and they don’t run.”

Trafton speculated that gulls likely became fouled in grease while looking for food. He said officers checked with the waterfront potato processing company Penobscot McCrum, but it did not appear the substance had come from its operations.

“If we knew what it was, it would give us a better idea of what the source was,” he said.

Gary Heitz of Avian Haven in Freedom took the three gulls captured by city police on High Street. Heitz said two additional gulls were collected Monday. Heitz said the gulls would be washed and, if it is determined they are not ill, returned to the wild. He said feathers would be taken as samples for testing.

Trafton said he planned to contact the Department of Inland Fisheries and Wildlife about the matter. He added that his officers would continue to assist residents when necessary.

“We’re not going to be picking up any more birds unless they’re up in somebody’s yard,” he said. “We’re not going to be chasing birds around.”