MINOT, N.D. - Warm weather that caused an upheaval of previously frozen ground has been determined to be the cause of the fire that raged for several hours March 22 at the construction site of the new Trinity Hospital complex on the southwest edge of Minot.

Fire department Inspector Dean Lennertz said his analysis of the fire scene concluded that shifting of the ground underneath the 24 1,000-gallon propane tanks at the location was sufficient enough to cause a leak, or a break, in one of the connections between the tanks. All of the tanks, which were spaced about three feet apart, were connected by copper lines.

"As the weather warmed up the ground became unstable," said Lennertz. "As the tanks moved it snapped a line or separated a line, broke a valve in some way and released the propane. That's what appears to have happened."

No one was injured as a result of the explosions or fire, primarily because construction had been halted the previous day due to a worker having fallen off the steel structure.

The possibility of the ground shifting was known to Yates Northwest, the contractor of the construction project. The explosions and resulting fire that was felt and heard throughout Minot occurred on a Friday. Earlier that week Yates had lifted the tanks to allow for the removal of ice underneath them. The tanks were stabilized by placing them on 4-foot-by-4-foot timbers.

Later that same week the temperatures rose again, leading to further shifting of the ground and resulting in slight movement of the tanks, which caused one or more leaks. The smell of escaping propane was apparent for an amount of time prior to the initial explosion.

At first a light breeze moved the propane cloud away from the scene but, eventually, the propane cloud found an ignition source.

"After the propane built up it reached a pilot light and ignited," Lennertz said.

The propane was being used as a heat source to allow for the curing of concrete poured during times of below freezing temperatures.

First responders blocked off a wide area around the construction site following the explosions. The fires were allowed to burn out rather than risk injury to firefighters who would face the blaze in a dangerous area.

Trinity Health was continuing to review the fire marshal's report Tuesday so had no immediate comment.

Construction remains halted as insurance investigators continue to examine the structures. Trinity Health spokesperson Karim Tripodina said it is not known how long the investigation will take but the project eventually will restart.

In the meantime, construction workers have been unable to retrieve their tools, harnesses, clothing and other gear left at the site. It has prevented some of them from seeking other jobs.

On March 21, an apprentice ironworker fell down an open elevator shaft and was hospitalized. When the fall occurred, a number of workers dropped their gear to respond. They were not allowed back onto the structure. When the site was then closed and safety inspectors called in, workers walked away leaving tools, thinking they would be back soon. Then the fire happened and "soon" never came.

Marc Jurek, business agent, Ironworkers Local 512, Mandan, said workers have been told they should replace their gear.

"Their tools are stranded out there but they have been given the opportunity to buy and get reimbursed by the subcontractors," he said.

However, younger workers, particularly apprentices, don't have the $500 to invest upfront or the borrowing ability to buy the tools, he said.

Those workers who can replace their tools, either by buying new or because they have extras, are finding work in and out of state.

Ryan Plummer of Fergus Falls, Minn., one of about 40 ironworkers who had been on the job, cleaned up old tools and took a job in Las Vegas. Still, he's concerned about getting his other tools back and worries about those workers not able to replace their gear.

"It's really affecting a lot of people," he said. "Our tools are like our children because we have a specialized trade."

It's also been stressful for workers because of the circumstances leading to the work halt, both witnessing a co-worker injured and watching the propane fire, he said.

Prior to the fire, workers had mentioned how scary it was to have the propane tanks so close to where they were working, Plummer said.

"They were a danger. It was ridiculous," he said. "We were so fortunate that we weren't on the job that day."