BOSTON — The FBI is offering a $20,000 reward for tips leading to the recovery of two N.C. Wyeth paintings stolen from a Maine collector two years ago in what officials on Tuesday called the most significant art theft in that state’s history.

Six oil paintings by Wyeth, the patriarch of a line of painters known for Maine seascapes, were taken in May 2013 from an apartment owned by Joseph Soley, a prominent real estate developer in the state. Soley has said his collection of Wyeth paintings had been independently valued at $50 million.

Four of the works were recovered in a Beverly Hills, California, pawn shop, according to Vincent Lisi, the special agent in charge of the Federal Bureau of Investigation in Boston.

Three men have pleaded guilty in federal courts in Maine and California to charges of trafficking in stolen goods for transporting the paintings. Lisi declined to say if any of the three were regarded as suspects in the theft.

“We’re still continuing to look for the other two [paintings], and we have reason to believe they are here in New England, possibly in the greater Boston area,” Lisi said.

The paintings were stolen from an apartment Soley had been using to store them, said Michael Sauschuck, Portland’s chief of police. The theft was first reported to Portland police on May 7, 2013, according to court documents.

During a sentencing hearing held last month for one of the man who transported the stolen paintings, Soley said he had been “shocked” by the theft.

“I had these most of my life and saved up for many, many years,” Soley said, noting that when he was a young man, he aspired to be a painter. “I was just fantastically disturbed to find someone would break in and take them.”

N.C. Wyeth is the patriarch of a family of painters that has become associated with Maine. His son Andrew Wyeth and grandson Jamie Wyeth have summered and painted in the Pine Tree State, and the Wyeths’ work often is exhibited by the Farnsworth Art Museum in Rockland, home to the Wyeth Center, and at the Portland Museum of Art.

Born in Needham, Massachusetts, in 1882, N.C. Wyeth got his start as an illustrator of books and magazines. He gained acclaim for painting seascapes after settling in Port Clyde, Maine, on the state’s rugged midcoast, where he lived until his death in 1945.

While the family is associated with Maine seascapes, the missing paintings are of other subjects. One, “The Encounter on Freshwater Cliff,” features a swordsman dressed in Renaissance-style clothing standing over what appears to be a slain foe. The other, “Go, Dutton, and that right speedily,” features men in medieval-looking attire peering out from a doorway.

It is not the only case of stolen artwork facing the Boston FBI office. The agency is still working to determine who stole $500 million worth of art from Boston’s Isabella Stewart Gardner Museum in a 1990 theft that stands as the largest art heist in U.S. history.

BDN reporter Mario Moretto contributed to this report.