The FBI announced on Tuesday it’s offering a reward of up to $20,000 for information regarding two N.C. Wyeth paintings that were stolen from a home in Portland, Maine.

Six Wyeth paintings were stolen from a business in 2013. FBI Special Agent Elizabeth Rivas in Los Angeles says the person responsible for taking the paintings remains a mystery.

Four of the six were recovered in a pawn shop in Beverly Hills, California, in December, and are estimated to be worth up to $2 million. But the remaining two were never found.

The FBI announced the reward on Tuesday and asked for the public’s help to find the missing paintings, identified as “The Encounter on Freshwater Cliff” and “Go Dutton, and that Right Speedily.”

Three men have been prosecuted in the theft, including Oscar Roberts, an aspiring rapper who used the stolen paintings to secure a $100,000 loan from a Beverly Hills pawn shop. In April, Roberts was sentenced to more than two years in federal prison.

The owner of the pawn shop, Yossi Dina, alerted police. The shop, The Dina Collection, is featured in a reality TV series, "Beverly Hills Pawn."

Lawrence Estrella, of Worcester, Massachusetts, pleaded guilty to interstate transportation of stolen property for transporting four of the six stolen paintings to California and was sentenced to more than seven years in prison.

Dean Coroniti, of North Hollywood, California, and formerly of Boston, pleaded guilty to possession of stolen property. He is scheduled to be sentenced in October.

FBI officials said they believe the two other paintings are somewhere in New England. However, David Bowdich, assistant director in charge of the FBI’s Los Angeles field office, also said they could be anywhere in the world.

"We are anxious to have this unique and irreplaceable artwork recovered and returned to its Maine owner," Thomas E. Delahanty, U.S. Attorney for the District of Maine, said in a statement.

The reward of up to $20,000 is being offered to anyone who can provide information leading to the recovery of the two paintings.

The Associated Press contributed to this report.