PHILADELPHIA - A heavy equipment operator with a lengthy rap sheet who is accused of being high on marijuana when a downtown building collapsed onto a thrift store, killing six people, surrendered Saturday to face charges in the deaths, police said.

Sean Benschop faces six counts of involuntary manslaughter, 13 counts of recklessly endangering another person and one count of risking a catastrophe. A warrant had been issued for his arrest and police had been searching for him.

Authorities believe the 42-year-old Benschop had been using an excavator Wednesday when the remains of the four-story building gave way and toppled onto an attached Salvation Army thrift store, killing two employees and four customers and injuring 13 others.

Deputy Mayor Everett Gillison said a toxicology report showed evidence that Benschop was high on marijuana. That finding, combined with witness statements and evidence from the scene, led to the decision Friday to raid his North Philadelphia home and later seek an arrest warrant, he said.

Benschop didn't return phone messages left by The Associated Press at numbers listed in his name.

Benschop, who also goes by the name Kary Roberts, has been arrested at least 11 times since 1994 on charges ranging from drugs to theft to weapons possession, according to court records. He was twice sentenced to prison in the 1990s after being convicted on drug trafficking charges. Benschop's last arrest, on a charge of aggravated assault, came in January 2012, but the case was dismissed for lack of evidence.

As the criminal investigation heated up, at least two survivors sued the demolition contractor and building owner, alleging gross recklessness at the job site.

The city, meanwhile, promised to crack down on the demolition industry.

"We can do much better," Mayor Michael Nutter said at a news conference Friday. "We will not accept the status quo in the face of this tragedy."

Nutter's reform plan for construction sites includes random drug testing on heavy equipment operators.

"If that's a factor here, that certainly takes things in a very different direction," he said hours before the charges against Benschop were confirmed.

The mayor also pledged to adopt tougher background requirements for demolition contractors, including information about each worker's experience, and more frequent site inspections when demolitions are underway.

His plan could run into resistance from builders who say they're already highly regulated.

"I think that before we do anything, before we rush to any judgment about how to fix what happened, we have to have all the facts," said Steven Lakin, executive managing director of the General Building Contractors Association, a trade group representing Philadelphia-area contractors. "Everybody wants to regulate demolition contractors, but I'm not so sure that's the answer."

Lawyers for the two survivors who have sued accuse demolition contractor Griffin Campbell - who has a criminal background and has filed for bankruptcy twice - of violating federal safety regulations. They say building owner Richard Basciano should have picked a more qualified and competent contractor to do the work.

No one answered the phone at a listing for Campbell on Saturday, and the voice mailbox was full.

Plaintiff Linda Bell, a 50-year-old mother of three, was shopping at the thrift store when the building came down on top of her. She fell into the basement and was covered by rubble for more than an hour.

"She's still shook up real bad, sore, swollen up," Bell's brother, Keith Bell, told the AP on Friday. She's also suffering mental anguish from "seeing other people getting killed," he said.

Construction engineers have said the thrift store should have been evacuated during critical phases of the demolition project next door.

The Salvation Army was concerned enough about the demolition that its attorneys reached out to a lawyer for building owner STB Investments Corp., a company linked to prominent businessman and developer Richard Basciano.

"There was communication between The Salvation Army and the attorney of the neighboring building's owner, pertaining to the demolition. The neighbor assured The Salvation Army that they would be taking proper precautions," Maj. Robert W. Dixon, director of operations of The Salvation Army of Greater Philadelphia, said in a statement Friday afternoon.

"These discussions were never finalized," he said.