The family of a deceased American artist is suing a retired therapist and Sotheby’s auction house for keeping and attempting to sell works they say were stolen from him.

Martin Ramirez Salinas and Maria de Jesus Reyes Ramirez Miller, grandchildren of Martin Ramirez, say that during the three decades in which he was involuntarily held in a Los Angeles mental hospital, his pencil drawings were confiscated from him. A psychologist visiting the hospital, Tarmo Pasto, discovered Ramirez’s talent and took 17 pieces to display publicly, later turning them over to Maureen Hammond, a psychologist who said she wanted to study them.

Ramirez’s grandchildren say she kept the art for four decades before contracting to sell them to Sotheby’s. According to the lawsuit, Sotheby’s contacted the Ramirez family, but when they demanded the art, Mrs. Hammond would not relinquish it.

The family is suing Mrs. Hammond and Sotheby’s for $3 million, requesting the works and an injunction against Mrs. Hammond and her counsel.

Mrs. Hammond’s lawyer, Rick Richmond, denied “absolutely” the family’s ownership claims. Mr. Richmond said Pasto distributed many Ramirez pieces, given to him either explicitly by the artist or through the purview of the hospital, to elite collections including the Guggenheim.

“Obviously, the estate’s lawyers must have believed Mrs. Hammond was an easier target,” Mr. Richmond said.

A representative of Sotheby’s, Lauren Goia, said, “Sotheby’s will maintain possession of the drawings until the court determines who has title.”