The Portland Museum of Art, Portland police and the FBI on Thursday announced the recovery of the last two of six N.C. Wyeth paintings stolen from Portland landlord Joseph Soley in 2013.

The two paintings, “Go Dutton, and That Right Speedily” and “The Encounter on Freshwater Cliff,” were recovered when an unidentified third party turned them over to retired Boston FBI agent Jim Siracusa.

Siracusa was first contacted by the third party in August, and after working out a deal that granted the person immunity from prosecution, the paintings were delivered undamaged to Siracusa in Burlington, Massachusetts, on Oct. 9.

“Both paintings were found in good condition, inside cardboard boxes, and in their original frames,” said Hank Shaw, special agent in charge of the Boston division of the FBI, which also covers Maine.

The value of the paintings is unclear. Estimates have ranged from hundreds of thousands of dollars into the millions.

N.C. Wyeth is the father of Andrew Wyeth, one of the best-known American artists of the mid-20th century, and the grandfather of Jamie Wyeth, a critically acclaimed artist who frequently paints scenes of the Maine coast. Soley says he got to know the Wyeths, particularly Andrew Wyeth, when he owned a summer home in Camden and the Wyeths lived nearby.

Law enforcement officials would not answer questions about the person who came forward or how that person came to possess the stolen art. They would only say that the works were probably transported to Massachusetts after the theft, and that the person who returned them was not connected to three people charged in December 2014 with trying to sell the four other Wyeth paintings taken in the burglary, then transported to Los Angeles. It wasn’t clear why it took nearly a month and a half to announce the recovery.

The FBI said no one has claimed the $20,000 reward it offered for the two paintings.

STILL A MYSTERY: WHO STOLE ART?

Before announcing the art recovery, experts at the Portland Museum of Art spent time authenticating the classic American works.

Now the six paintings will be the subject of a special exhibit by the museum, “The Great N.C. Wyeth Caper: Paintings by America’s Storyteller,” along with a seventh N.C. Wyeth canvas from the same period on loan from a separate owner. The exhibit opens Saturday and will run until Dec. 6.

Shaw, the FBI agent, said no arrests have been made in the burglary and thefts, but the investigation is continuing.

Steve Kurkjian, a Boston-based investigative reporter and author who has spent the past quarter-century reporting on the unsolved theft of 12 works from Boston’s Isabella Stewart Gardner Museum in 1990, said the FBI has offered a similar reward in the Gardner heist.

In the case of the Wyeth paintings, Kurkjian speculated that they were returned out of desperation to avoid prison, not goodwill.

“The (perpetrators) are coming in because they feel the heat of the FBI,” Kurkjian said. “I don’t think they came in from the goodness of their heart.”

FBI SAYS REWARD OFFER JUSTIFIED

The FBI offered the $20,000 reward in August, a month after a New Hampshire man was convicted of illegally transporting across state lines the four other N.C. Wyeth paintings stolen from Soley’s downtown apartment.

Lawrence Estrella, 65, of Manchester pleaded guilty to the felony count July 14 and was sentenced to seven years and eight months for taking the four paintings to California in the trunk of a green Mercedes. Estrella has a lengthy criminal history that includes robberies and breaking and entering.

The paintings were taken from an unoccupied apartment at 18 Monument Square in Portland. The theft was discovered on May 7, 2013, when a Soley family member went to the apartment.

The FBI said the theft was probably the most significant art heist in Maine history and justified the offering of reward money.

Court documents in Estrella’s case listed the value of the four paintings at $1 million, although Soley has said it’s difficult to estimate the value and that they could be worth up to $50 million.

TWO OTHERS POSSESSED STOLEN ART

On Nov. 23, 2014, police located Estrella’s green Mercedes in the parking lot of a Comfort Inn in North Hollywood, California, and began surveillance of Estrella, court records said. Less than a month later, on Dec. 19, 2014, police recovered four of the six stolen paintings from a pawnshop in Beverly Hills.

Also implicated in the criminal case was aspiring rapper Oscar Roberts, 37, of Los Angeles, who was sentenced to 28 months in federal prison for pledging stolen property as security for a loan.

Roberts took the paintings to the Dina Collection, a Beverly Hills pawnshop that’s featured on “Beverly Hills Pawn” on the Reelz network, for a $100,000 loan.

Dina Collection owner Yossi Dina called police after suspecting the art was stolen.

A third man, Dean Coroniti of North Hollywood, pleaded guilty to possessing stolen art and awaits sentencing.