Federal workplace safety officials have visited the scene of Wednesday’s deadly construction site fire in Denver’s North Capitol Hill neighborhood and launched what is expected to be lengthy investigation into what happened there.

“I can tell you that we are working with the fire department investigating this fire and looking at (the contractors’) overall safety and health program,” Herb Gibson, Denver area director for the Occupation Health and Safety Administration, said Friday. He estimated it would be months before conclusions are reached.

Local labor organizers, meanwhile, question the safety protocols at the site where crews were erecting the five-story Emerson Place Apartments complex before the devastating fire erupted, killing two workers and injuring six others. Those organizers say there is evidence at least one subcontractor employed workers who were paid under the table and not signing daily work logs, making it hard to account for everyone on site.

The Southwest Regional Council of Carpenters responded to the fire scene Wednesday afternoon to assist workers who had evacuated, union special representative Mark Thompson said.

“When I approached the foreman for United Builders Service, I asked him how many workers he had on his crew. He looked me straight in the eye and said, ‘I don’t know,’ ” Thompson said at Friday’s rally. “This is unacceptable. We pray there are no other workers missing, but how can the professionals be certain without an accurate count to start with?”

Two bodies have been found. A GoFundme page set up Thursday identified Roberto Flores as one of the dead, though that has not been confirmed by officials.

Originally three missing persons reports were filed, but officials determined two reports concerned the same man, Denver Fire Department Capt. Greg Pixley said. There is no indication anyone else is missing, Denver police say.

United Builders Service issued the following statement: “We will be eternally grateful to our heroic superintendent who risked his life by running back into the burning building up to the third floor to make sure every last one of our workers were accounted for and safely outside. Our thoughts and prayers are with all those affected by this terrible incident.”

But USB officials declined to respond to the allegation the company used workers who were being paid under the table.

OSHA had not fielded any complaints regarding safety at the Emerson Place work site prior to Wednesday, Gibson said. The project’s general contractor, Frederick-based Vertix Builders Inc., has a license in good standing, city officials said this week.

There were approximately 50 workers on the site representing eight to 10 trades Wednesday, Vertix officials said. In a written statement issued Friday, the company said it provided Denver fire officials with a full list of workers who were working Wednesday shortly after emergency crews arrived. The list included the names of the two men killed.

The company strictly adheres to labor laws, including those regarding pay for workers and workplace safety regulations, the statement says.

A site-specific safety plan was in place for the Emerson project, and all egress pathways at the building had been evaluated by an independent safety auditor and found to be compliant with guidelines prior to the fire. The statement called allegations that off-the-books workers were on the job site “patently false.”

“Vertix Builders places the highest value on the health and safety of our employees, subcontractors and the people who live and work in the vicinity of a Vertix Builders project,” the statement reads. “We are committed to determining the facts and working with investigators to establish the cause of this incident.”