The accident happened about 9:45 a.m. in Somerset Township, about 30 miles south of Pittsburgh, minutes after the van had left a group home in Bentleyville.
"I just think it's one of those terrible, sad freak accidents," said Lynne Loresch, executive director of the Mental Health Association of Washington County, the group home operator.
Loresch said the van was the second of two taking group home residents to the zoo. The first van arrived safely, but the group was told to return home, where they learned of the crash.
Three residents of the group home and two staff members were killed. The six other people in the van - all residents - were taken to hospitals. The tractor-trailer driver was treated at a hospital and released.
Authorities said they did not believe the tractor-trailer had been speeding. State transportation officials hauled it from the scene and planned to inspect it.
The tractor-trailer is owned by Stocker Trucking Co. in Gnadenhutten, Ohio, and was driven by Stephen Rouse, 44, of Uhrichsville, Ohio. A man who answered the phone at Stocker Trucking declined to comment.
The van was pinned against a cinderblock storage building owned by a home gas- and oil-delivery business; a corner of the building partly collapsed.
The van was knocked onto its side and pushed roof-first into the building, causing the vehicle's roof to cave in, state police Trooper Frank Lewis said.
Two of the dead were removed by paramedics shortly after the accident, but the bodies of the driver and two residents had to be cut from the wreckage, Washington County Coroner Tim Warco said.
Killed were the driver, Sheryl Maiolini, 53, of Charleroi; staff member Mary Watkins, 43, of Ellsworth; and residents John Maise, 61, Richard Paquet, 43, and Julie Hugus, 41.
Maiolini was a five-year employee with a "superb" driving record, Loresch said.
The injured residents included three men and three women, who range in age from 48 to 58 years old.
Two of them had returned to the group home by late afternoon, Loresch said. The others were expected to survive and had injuries that included head, neck and spinal injuries and broken bones, Loresch said.
Loresch said people in the van that crashed are from the facility's enhanced personal care home, whose residents aren't locked down around the clock.
Most of the home's residents have schizophrenia or bipolar disorders. Some are there voluntarily, while others had been at Mayview State Hospital and are making the transition toward more independent living, Loresch said.
Authorities had no immediate theories about why the van pulled into the path of the truck on state Route 136.
Jeff Breen of the Pennsylvania Department of Transportation's Washington County maintenance office said state officials were aware of safety concerns at the intersection of the two-lane highway and Brownlee Road, from which the van pulled out. The department had studied the intersection and planned an upgrade, he said, though he wouldn't provide specifics.
"We will look at it again and see if we will move this project ahead of other projects," Breen said.
Motorists on Brownlee Road, which travels north and south, must stop at the intersection of Route 136; motorists on Route 136, which travels east and west, do not have to stop.
Before Thursday, there had been 15 crashes but no fatalities at the intersection over the past five years, said Rich Kirkpatrick, a PennDOT spokesman.
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Associated Press writers Ramesh Santanam and Ramit
Plushnick-Masti in Pittsburgh contributed to this report.
