By Dave Warner

PHILADELPHIA (Reuters) - A crane operator who police said tested positive for marijuana use on the job was arrested on Saturday on charges of involuntary manslaughter stemming from a building collapse in Philadelphia that killed six people and injured more than a dozen.

Sean Benschop, 43, accompanied by his lawyer, turned himself over to police to face six counts of involuntary manslaughter and 13 counts of endangering another person, Philadelphia police Sergeant Mike Veasey said.

Police said Benschop was assisting with a building demolition on Wednesday when a wall of the structure collapsed onto an adjacent building that housed a Salvation Army thrift store filled with shoppers and staff.

Toxicology reports on blood samples taken from Benschop after the accident showed he marijuana in his system, police said.

Benschop's attorney, Daine Grey Jr., told reporters outside the police station that his client was not at fault for the collapse.

"This was an accident, but Mr. Benschop is not responsible," Grey said in remarks aired by a local CBS television affiliate. "We believe in time the facts will show that he is not responsible and that the responsible party will be held accountable."

The four-story building that was under demolition on Market Street in Center City was once home to a store that sold X-rated books and videos, according to authorities.

Philadelphia Mayor Michael Nutter on Friday recommended random drug tests for heavy equipment operators and other safeguards as part of a sweeping set of reforms after the building collapse.

The city will also require demolition contractors to provide a site safety plan to protect pedestrians and adjacent property.

Attorneys representing survivors indicated they intend to sue the building owners, Richard Basciano and the STB Investments Corp, and the Philadelphia-based demolition company, Griffin Campbell Construction.

Among those injured was Nadine White, 54, a mother of three and a clerk who was working at the thrift store when chunks of concrete came raining down on her, burying her in the rubble, her lawyer Robert Mongeluzzi said in preliminary court papers.

Another was mother of three, Linda Bell, 50, who was shopping in the store when the collapse occurred, her lawyer Joseph Marrone said. She was buried under the rubble for about an hour before being rescued.

(Reporting by Dave Warner; Writing by Colleen Jenkins; Editing by Peter Cooney and Gunna Dickson)