PHILADELPHIA -- A two-story home being demolished in Philadelphia partially collapsed Monday. One of two contractors was killed in the incident just one day shy of the fifth anniversary of another building collapse in the city that killed six people.

Two men were working on the building shortly before 11 a.m. when the rear of the property collapsed, trapping them, police said. One worker was able to escape, but the other, a 60-year-old, remained trapped under the rubble and was pronounced dead shortly after 11 a.m.

Aerial footage from CBS station KYW-TV showed firefighters digging through the rubble.

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Fire commissioner Adam Thiel called it "a very difficult afternoon here, for our responders as well as everybody who's affected by this incident." He said the building "is still very unstable, which is why we're not inside of it anymore."

The city Department of Licenses and Inspections was at the scene, along with the federal Occupational Safety and Health Administration, to investigate the cause of the collapse.

City property records listed the home as "structurally compromised," and the demolition permit said the masonry building was to be completely demolished "by hand-method only" to "resolve dangerous case."

Most demolitions in Philadelphia are carried out by hand, said Licenses and Inspections Department spokeswoman Karen Guss. She said cranes and backhoes are prohibited, although hand tools may be used.

The department visited the site of the demolition several times and "had no reason to believe that building was being demolished by prohibited means," Guss said. Philadelphia, she said, has "probably the most stringent demolition safety laws in the nation."

The collapse happened one day before the fifth anniversary of the collapse of another building in the city that left six people dead and 13 injured. A towering brick wall left unbraced during a demolition project fell and crushed an adjacent Salvation Army thrift store on June 5, 2013.