DENVER -- Safety officials are investigating whether plumbers failed to properly stabilize a trench that caused a Victorian home to partially collapse, injuring two workers Wednesday night.

"We're trying to determine the (collapse) cause and if any standards were violated," Herb Gibson, area director for the federal Occupational Safety and Health Administration, told TheDenverChannel.com Thursday.

Denver fire and building officials are working with OSHA on the investigation, which will take about a month.

Meanwhile, OSHA inspectors are trying to confirm the name of the plumbing contractor, initially identified as United Plumbing and Heating.

"We're still working on the exact name of the company. It's a very small employer," Gibson said.

OSHA officials inspected and photographed the collapse scene Thursday at the Victorian home in the 2700 block of Stout Street, Gibson said. The plumbing firm had no prior OSHA inspections or violations, he added.

A key issue will be whether the plumbing firm complied with OSHA trenching and structural stabilizing regulations, which sometimes require reinforcing trenches with timber to protect workers from cave-ins, Gibson said.

The two plumbers were installing a 1-inch copper water line from a utility connection in the front yard in a trench about 30 feet long that ran between the two homes, said Denver fire spokesman Lt. Phil Champagne. The plan was to run the water line into the crawl space of the Victorian that partially collapsed.

What posed the problem is the workers dug a trench about 18 inches wide, between neighboring Victorians that are only about three feet apart, Champagne said. The trench ran as deep as 6 feet in some places.

The homes were built in the 1880s and the collapsed Victorian had an aging foundation made of stones, brick and mortar, he said.

"The trench undermined the foundation and the foundation gave way. That's what created the collapse," Champagne said.

Two workers were trapped in the collapse. One worker extricated himself.

But the second buried worker required a dangerous rescue with a dozen firefighters in the trench carefully removing stones and bricks while hurrying to shore up the unstable house, he said.

"We were worried about our own guys, too," Champagne said. "God forbid the whole thing collapses."

Fortunately, the fire agency was equipped with emergency aluminum jackets and beams, purchased in preparation for the city hosting 2008 Democrat National Convention, Champagne said. This allowed a trench rescue team to swiftly stabilize the shaky structure.

"This guy was very lucky, because he was buried in the trench with the debris on top of him," he added.

Champagne said the two injured workers are expected to be OK.

The woman in the neighboring house voluntarily evacuated her home at the urging of fire officials.

"We're worried about the house collapsing (completely) into the other neighboring house," Champagne said. "There's about a 14-foot masonry chimney that's being held up just really by nothing. And if it goes, it's going to take a lot with it."