A severely retarded man who could not hear or talk died Friday after being left in a van on a 99-degree day by staff members from his city-contracted group home who did not notice he was missing for more than five hours, officials said last night.

Patrick Dutch, 41, who lived in a group home on Quincy Place NW operated by D.C. Health Care Inc., was pronounced dead shortly after 9 p.m. Friday, said Jonathan L. Arden, the city's medical examiner. An autopsy was performed Saturday, but the official cause of death still has not been determined, Arden said.

D.C. Mayor Anthony A. Williams said he had ordered the police, human services and health departments to investigate Dutch's death. The U.S. Justice Department already is investigating the quality of care at the District's government-funded facilities for the retarded after Washington Post articles published in March found 350 documented cases of abuse, neglect and profiteering in the city's 150 taxpayer-financed group homes for the retarded.

"I want to assure the family of Mr. Dutch as well as the families of other mentally challenged persons that swift action will be taken as appropriate once these investigations [into Dutch's death] are completed," Williams said. "We have a sacred trust to ensure the well-being of our most vulnerable citizens."

A van driver and an escort drove Dutch and seven other clients at 11:30 a.m. Friday from the group home on their regular daily trip to a day program at PSI Associates Inc., in the 700 block of M Street SE, according to interviews with city officials and care providers. As several clients were escorted into the facility, one "started running," distracting the driver. No one noticed that Dutch had not left the van, said Gracy Stephen, co-owner of D.C. Health Care Inc., operator of the group home where Dutch lived.

The van returned to the group home after dropping off a client at another day center and returned to the PSI Associates facility about 3:30 p.m. Staff members didn't realize until 4:50 p.m., after they had returned to the group home, that Dutch was missing, officials said.

A search ensued, and the driver returned to the day program on M Street SE, thinking that Dutch might have wandered off, officials said.

At 7:05 p.m., while searching a wooded area at 12th and M streets SE, officials said, the driver stopped the 15-passenger van and noticed that Dutch was in the long rear seat, curled up. He was pronounced dead at the scene, Arden said.

A city official said that the van's air conditioning would have been turned off and that the windows probably were shut while the van was parked.

"He had a habit of going back to the back seat and lying down," Stephen said. "Nobody realized that he had done that at that time."

Dutch's brother, Dennis Dutch, 46, said his relatives could not understand how Patrick was left in the van.

"He's tall," said Dennis Dutch, who said Patrick was 5-foot-11. "We could not understand why they couldn't see him back there."

Based on first reports, "it's sort of hard to understand" what happened and "how this guy was left in the van without someone realizing it," said Paul Offner, head of the city's Medical Assistance Administration, which contracts with private operators for the group homes.

Arden said officials are considering heat and Dutch's several long-standing illnesses as potential causes of his death, as well as "other things that are not suggested by his background or his history."

Officials from the Department of Human Services have regularly described D.C. Health Care Inc. as one of the best group-home contractors. The 18-year-old company receives federal Medicaid funds to provide housing and transportation to more than 60 people, city officials said. Stephen said the company runs seven group homes in the District.

The Mental Retardation and Developmental Disabilities Administration, a branch of DHS, gave the company awards in 1987 and 1994 for quality of care, Stephen said.

A spokeswoman for the Department of Human Services said the agency has a responsibility for monitoring the progress of people assigned to such homes, and for general case management, but did not operate the group homes.

Patrick Dutch was born with cerebral palsy and at first could not walk on his own, his brother said. He was severely retarded and operated at the level of a 3-year-old. He could not hear or talk and was partially blind after the removal of a cataract two years ago, Dennis Dutch said. In addition, he was diabetic and had hypertension, his brother said.

Born in Southeast Washington, Patrick Dutch entered a District-run asylum called Forest Haven in Laurel at age 4 and remained there until it closed, his brother said. Dutch was then brought into a medium-supervision home on Georgia Avenue NW. In January 1993, he was moved to the home on Quincy Place.

Dutch was the youngest of nine children and is survived by his mother, three brothers and three sisters, his brother said.

"It sounds like negligence," Dennis Dutch said. "They had too many opportunities to perform the simple procedure of a head count, or checking the bus to make sure there was not one aboard. . . . They're being too careless."

Stephen called the death a tragic accident. "It's one of those grave human errors," she said.

Researcher Heming Nelson contributed to this report.