The 40-by-30-inch de Kooning painting was gone. Investigators believe the man cut it from its frame and rolled up the canvas and stuffed it under his heavy blue coat while the woman distracted the guard, who could not see the gallery from the landing where they had talked. The two drove away in a rust-colored two-door car. At the time, “Woman-Ochre” was valued at $400,000.

It was a highly unusual crime. Despite the depiction of art heists in movies and television, a vast majority involve works taken from storage areas by employees or people in a position of trust. With few leads beyond a description of the thieves — and sketches of the pair prepared by an F.B.I. artist based on witness accounts — the crime became an enduring mystery.

“We’re looking at everything — absolutely,” said Brian Seastone, the University of Arizona police chief, when asked about whether investigators were looking into the possibility that Jerome Alter and his son, Joseph, were involved in the theft. He would not say what other avenues were being pursued. The university Police Department is assisting the F.B.I. with the case; Chief Seastone was involved in the initial investigation in 1985, when he was a police officer with the department.

Jill McCabe, a spokeswoman for the F.B.I. in Phoenix, would not comment other than to say that the bureau has “an active and ongoing investigation into the theft.” The inquiry is being conducted by agents in the Tucson office, with the assistance of the agency’s Art Crime Team in New York.

The sketch of the female suspect — described at the time of the theft as being between 55 and 60 years old — bears a resemblance to Mr. Alter, who was known as Jerry and was then 54. And the sketch of the young man — described at the time as between 25 and 30 years old – bears a resemblance to his son, Joseph M. Alter, who was then 23. At around that time, the Alters had a red two-door Nissan sports car, according to a family member, a family friend and owner of a gas station less than a mile and a half from the Alter home.

Witnesses described the woman as having blondish-red shoulder-length hair, covered by a scarf of the same color. She wore tan polyester bell-bottom slacks and a red water-repellent winter coat.