OAKLAND — The state’s pesticides control board is expected to launch an investigation today into the spraying of a weedkiller on a dam on Messalonskee Lake.

Ed Pearl, a former director of Friends of Messalonskee Lake, said that he was driving past the dam on the north end of the lake Friday afternoon when he saw a man spraying a liquid in a blue tank sprayer on weeds growing out of the dam’s boards.

“I’d say more was probably going in the water than was going on the weeds,” Pearl said.

Pearl said that the man appeared to be working on behalf of the company that owned the dam, and he had keys to a fence and outbuilding on the dam.

The dam is owned by Essex Hydro Associates, of Boston, according to records at the town office.

Two calls to Essex Hydro Associates were not returned Monday.

Pearl said he reported the incident to the Oakland Police Department and to the Maine Board of Pesticides Control.

The danger to humans was unclear, but the waterway is important to the area as a recreational area and contains fish that residents eat, Pearl said.

“There are a fair amount of trout in that area. There are people that kayak in it. There are people that swim it in. It goes into the Kennebec River.”

Ray Connors, of the state’s Pesticides Control Board, said Pearl’s complaint triggers an investigation by the state, during which Pearl, the person who applied the substance and the dam owner probably would be interviewed.

“Complaints are a priority,” Connors said. “Whenever possible, the BPC tries to respond to complaints the same day they are received.”

Connors said the board’s inspector was taking care of another case Monday, when the complaint was made, but probably would initiate his investigation today.

In addition, the site could be tested for physical evidence of the substance sprayed.

“The determination of whether or not to sample will be based in large part on the information obtained through the follow-up inspection process,” Connors said.

The investigation and resolution process usually is completed within a year, Connors said, although more involved cases can take several years.