Correction: An earlier version of this article misidentified the consulting firm hired by WMATA to help with an investigation of the incident. It is K&J Safety and Security Consulting Services, not KNJ. This version has been corrected.

A worker killed last weekend during a rail-replacement project on Metro’s Red Line suffered a fatal chest injury in a rapid, out-of-control sequence of events that began when a hose filled with hydraulic fluid sprung a leak, transit officials said Thursday.

The flammable fluid, spraying like mist from an aerosol can out of a “pinhole” in the hose, was instantly ignited by heat from two lengths of rail that had been welded together moments earlier, Metro General Manager Richard Sarles told reporters at the transit agency’s board of directors meeting.

The flash fire, in a subway tunnel near Union Station, caused workers to flee, including 41-year-old Harold Ingram, of Blackstone, Va. As he was running, Ingram was struck in the chest by a 40-foot-long piece of iron rail that was attached to the arm of a crane and moving like a one-ton projectile, Sarles and other officials said.

He died in an ambulance on the way to a hospital.

“Our hearts go out to the family,” Sarles said. “It’s tragic. It hurts us all as a family to see someone die that comes to work to just do a job and leave at the end of the day.”

In the transit agency’s first detailed public account of the accident, which occurred a few minutes after midnight Sunday, Sarles and Metro’s assistant general manager, Robert Troup, described what investigators think happened in the tunnel.

A vehicle called a “prime mover” was outfitted with hydraulic equipment that presses new pieces of rail together on the track bed and welds them in place. About 70 feet away, a small crane was being used to pull up an old piece of rail from the track bed. When the fireball suddenly erupted, Ingram was standing between the prime mover and the crane.

Sarles said Ingram ran away from the fire, in the direction of the crane. At the same time, the crane began to back up, with its arm still attached to a piece of rail that had yet to be unfastened from the track bed, Sarles said.

“That crane had locked onto a piece of rail in preparation for pulling it out,” Sarles said. As the crane backed up, “what it did was, it really pulled hard” on the rail. “And think about anything you pull hard on, and it’s suddenly released. It popped loose. And in popping loose, that is when it struck and fatally injured the employee.”

Ingram was an employee of Holland Co., a Crete, Ill., firm contracted to do welding work for Metro. Citing “the potential for litigation,” Sarles declined to discuss who was responsible for the leaking hydraulic fluid that caused the prime mover and its automated welding equipment to become engulfed by a flash fire.

Workers quickly put out the fire with hand-held extinguishers, he said.

Sarles said authorities have not yet determined why the crane moved backward with its arm still attached to the rail — whether it moved on its own or the crane operator was responsible.

About 15 people were working in the area when the accident occurred, a Metro spokesman said. Sarles said the investigation is being handled by Metro safety officials and supervised by the Tri-State Oversight Committee, which monitors safety and security on the Metrorail system in Maryland, Virginia and the District.

The National Transportation Safety Board normally would take part in such an investigation. But because of furloughs stemming from the partial federal government shutdown, the board is not involved in the inquiry.

To help with the investigation, Sarles said, Metro has hired K&J Safety and Security Consulting Services Inc., of Cantonment, Fla.