Wagenhoffer twice inspected the demolition of the building on Market Street that collapsed last week, hitting the Salvation Army Thrift Store next door. Three employees and three people who were at the store to shop or drop off donations were killed.
The inspector, 52, was found dead inside his truck of an apparent gunshot wound to the chest Wednesday.
"It was my fault. I should have looked at those guys working, and I didn't," Wagenhoffer said in a video message he recorded on his cellphone just before apparently shooting himself, police said.
"When I saw it was too late. I should have parked my truck and went over there but I didn't. I'm sorry," he said.
Wagenhoffer said he was so upset about the deaths that he couldn't sleep, the Philadelphia Daily News reported Friday.
Mayor Michael Nutter's chief of staff, Everett Gillison, said Wagenhoffer was not to blame for the accident.
"I will state right here, right and now, this man did nothing wrong," he said Thursday. "The department did what it was supposed to do under the code that existed at the time, and we are proud of the department and its employees -- period."