PHILADELPHIA » A felon who was allegedly high while operating demolition equipment when a downtown building collapsed and killed six people will be charged with involuntary manslaughter, a top city official said today.

Sean Benschop, 42, faces six manslaughter counts along with six counts of risking a catastrophe, six counts of reckless endangerment and other charges, Deputy Mayor Everett Gillison told The Associated Press.

Authorities believe Benschop had been using an excavator Wednesday when what was left of the four-story building gave way and fell on top of a neighboring Salvation Army thrift store, killing two employees and four customers, and injuring 13 others.

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

"The D.A. has approved it (his arrest), and my police officers are out looking for him as we speak," said Gillison, the deputy mayor for public safety.

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, for aggravated assault, came in January 2012, but the case was dismissed for lack of evidence.

Benschop did not return phone messages left at numbers listed in his name, though he told The Philadelphia Inquirer on Thursday that he couldn’t comment because of the investigation.

The victims include a pair of 24-year-old artist friends shopping at the store and a newly engaged woman working her first day there.

Video shot of the scene days before the fatal collapse show bricks falling onto a sidewalk, which remained open to pedestrians, as a worker used heavy equipment to take out a front wall.

Some blame has been lobbed at demolition contractor Griffin Campbell, whose background includes arrests for drugs, assault and insurance fraud, along with two bankruptcy filings. He was being paid $10,000 for the job, according to the demolition permit.

Campbell violated several federal safety regulations, while building owner Richard Basciano should have picked a more qualified and competent contractor to do the work, said lawyer Robert Mongeluzzi, who represents injured store worker Nadine White in a lawsuit filed today against the pair.

"From what we can understand, given (Campbell’s) checkered past, and what appears to be a total lack of experience and know-how, we believe that was a grossly negligent selection," said lawyer Robert Mongeluzzi, who won court permission today to examine the debris after city officials finish their investigation.

The collapse has brought swift and mounting fallout in a city where demolition contractors are lightly regulated. Officials have begun inspecting hundreds of demolition sites citywide, and a city councilman charged that dangerous, under-the-radar tear-downs are taking place throughout Philadelphia.

The city is also preparing to implement sweeping changes in its regulations of building demolition, Mayor Michael Nutter said today, adding that every active demolition site is being inspected for safety.

The mayor appeared forlorn at the afternoon news conference at City Hall, apologizing to the victims’ families for the deaths and promising the city would do better.

"We lack the resources to have a police officer on every corner, or L&I (License and Inspections) at every construction site every hour of the day, (but) we can do much better," Nutter said. "We will not accept the status quo in the face of this tragedy."

Nutter’s reform plan for construction sites that includes the random drug testing of heavy equipment operators.

"If that’s a factor here, that certainly takes things in a very different direction," Nutter said hours before the charges 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 under way.

Several homicide prosecutors and police department crime scene investigators were on the scene this week when a steel beam from the site was removed.

Construction engineers have said adjacent buildings should be evacuated during critical phases of a demolition project. The other victims also include two immigrants from Africa, a 68-year-old man from Liberia described as a devoted husband and Salvation Army worker, and a 52-year-old woman from Sierra Leone who had nine children and loved to hunt for bargains.

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 this afternoon.

"These discussions were never finalized," he said.

Rubinkam reported from northeastern Pennsylvania. Associated Press writers JoAnn Loviglio, Kevin Begos and Keith Collins contributed to this report, along with AP’s News and Information Research Center.