Sources involved in the investigation say the explosion was likely caused by a mechanical failure in a generator stored inside a metal trailer on the site.
It happened after 3 p.m. Wednesday on the 5000 block of East Flat Rock Road, a piece of land that runs between the Schuylkill River and Manayunk Canal.
According to a Facebook page for Quandel Performance Construction, its crews had been working on a project on the Flat Rock Dam in the Schuylkill River.
"It's a storage trailer, probably about 8x10, and it's just used for storage, nothing else," said Philadelphia Fire Battalion Chief Derek Bowmer.
Officials: Generator explosion injures 6 people at construction site in Manayunk
The victims are all men and range in age from 22-48. They were taken to Temple University Hospital.
As billowing smoke poured out of the metal trailer along the Schuylkill River, workers covered in soot looked for help.
Medics raced those with life-threatening injuries to the hospital.
Robert Edelstein saw the smoke in the distance while he was on the river.
"Right when we got there, it was pretty raging and a lot of smoke and flames coming from it. We did not hear the explosion but it definitely got a lot more where we could smell the diesel fuel," he said.
The location of the explosion did make this a difficult fire to fight.
"We have issues of laying our hose. It's a long stretch," said Bowmer.
As the fire marshal continues to investigate the fire, a day later, people in Manayunk are still rattled.
"Freaky. Not something you want to see in Manayunk. We love the canal area so I'm sure everyone here is feeling a little unsafe," said Alyssa Hall.