SEARSPORT, Maine — Two groups opposed to the construction of a massive propane tank and terminal project in Searsport have put the Army Corps of Engineers on notice, saying that they will file a lawsuit against the agency if it does not immediately withdraw the permit it issued last year.

“The environmental assessment done last year was not sufficient,” Steve Miller of the Islesboro Islands Trust said Saturday. “We all want that the Army Corps of Engineers will withdraw the permit. The ball is in their court.”

His nonprofit land trust and Thanks But No Tank, a group of citizens opposed to the proposed liquid propane gas project, on Thursday filed a 60-day notice of intent to sue the federal agency. In part, Miller said, the reason he believes the environmental assessment that already was done isn’t good enough is that new information has emerged leading opponents to question why Denver-based DCP Midstream wants to build a terminal to import propane to Maine.

The controversial $40 million complex would be built in the Mack Point industrial zone and include a 14-story, 23-million-gallon liquid propane storage tank. It has been green-lighted by agencies including the Maine Department of Environmental Protection, the Coast Guard and the Army Corps. It now awaits a decision from the Searsport Planning Board, whose volunteer members held several long evenings of public hearings over the past few months to gather information and opinion in advance of their vote.

The planning board will begin its own deliberations this week with 6:30 p.m. meetings scheduled Wednesday and Thursday at Union Hall in Searsport. During the deliberations, people who are not planning board members may participate only if requested by the board, longtime chairman Bruce Probert said Saturday.

Miller said that another lawsuit, this one against the Maine DEP, is awaiting a decision by the Maine Supreme Judicial Court. He said he wants to know why DCP Midstream wants to build a facility to import propane to Maine, given what he called a “glut of propane” on the market domestically.

“There is so much propane that many companies in the United States … are exporting propane,” Miller said. “We don’t need an import facility. The only thing we can guess, and it is speculation, is perhaps they’re going to convert it to an export facility. If it were an export facility, everything changes.”

Exporting propane from Searsport would raise concerns about increased truck and tanker traffic, he said.

When, during a planning board public hearing in January, another opponent raised the idea that the company could export propane from Searsport, DCP Midstream’s Maine attorney, Jamie Kilbreth, decried the suggestion.

“Are you seriously suggesting that DCP moves its propane from Texas, Oklahoma and Colorado to Maine to export rather than Houston?” he asked then. “It cannot be used as an export terminal as it’s designed and proposed. This idea is just a nonstarter.”

Efforts to contact a DCP spokesperson this weekend were unsuccessful.

Probert said the planning board is scheduled to meet and deliberate the propane project this week as well as April 3, 10, 11, 17 and 18, if necessary. He expects the board to make a decision this spring, regardless of the ongoing lawsuits.

“We continue to move forward until a judge or court tells us to stop,” he said. “There’s a light at the end of the tunnel, and it’s not a locomotive coming at us. There will be a finite resolution.”