It began around 11:13 a.m. in the 1200 block of S. 22nd Street.
That's where police officers and firefighters responded for a possible neighbor dispute and a strong smell of bleach and ammonia.
Naomi Stewart called 911. She lives next door to the home, and says the smell was overwhelming.
"I was afraid my house was going to blow up," she said.
Chopper 6 video from hazmat incident, police response in South Philadelphia
But what she described to police prompted a response from counter-terrorism.
"The guy had a bunch of deliveries of chemicals the other day, and today the smell was coming through my house," she said. "It was so nauseating."
Stewart said the man had been screaming, saying someone was in there.
"He had guns, that someone was trying to kill him. It's just been non-stop," she said.
It turns out the man was in the process of being evicted from the home. Neighbors say it's been going on for months.
Recently, they've heard drilling come from inside and police have been called repeatedly.
"Because we have a prior history with this individual, maybe stable, unstable, we took the proper precautions. Handled it like a barricade situation," said Capt. Kenneth McKinney of the 17th District.
The man was not inside the house when police went in, but investigators say it looks like a hoarding situation.
Their crews didn't detect anything hazardous that would pose a danger to the neighborhood but they did find chemicals.
Firefighters remained on the scene to remove those chemicals.
L&I put up a notice on the building saying that it's unfit for human occupancy.
Police say if the man comes back to the house he could be arrested.