City officials are investigating a fire that burned for nearly six hours Monday night on Kapahulu Avenue after a milling machine struck and broke a shallow gas line and sent flames shooting out about 20 feet, injuring two workers.

The milling machine was digging down 9 inches to remove old asphalt and struck the gas line, which was buried just 7 to 8 inches deep, said Solo Pamatigan, project manager for Road Builders, whose machine hit the line. The company is contracted by the city to repave Kapahulu Avenue.

Typically the depth for all utilities is 3 feet, city Deputy Director of Design and Construction Mark Yonamine said at a news conference Tuesday afternoon. A city spokesman later said the line was shallow because the gas company had to navigate around an existing electrical line to install the gas line.

Honolulu fire spokesman Capt. David Jenkins said HFD got the call at 9:24 p.m. of a fire near the intersection of Kapahulu Avenue and Date Street, the first fire company arrived at 9:31 p.m. and there were "large columns of flames jetting about 20 feet out in all directions," he said.

Two workers were sent to the hospital. Pamatigan said the operator fell 8 feet from the machine and was uninjured but was taken to the hospital to be checked out and released that night. The other worker, who was closer to the gas line, remained hospitalized Tuesday, he said.

Hawaii Gas officials arrived at 9:55 p.m. and reported it would take five hours to shut the gas off due to multiple lines feeding the leaks, HFD said. The Fire Department reported that the fire was extinguished at 3:14 a.m. Tuesday.

A Hawaii Gas spokesman said a blade from the machine hit the gas line and that a spark from the impact likely ignited the fire.

The city and the contractor said there is a system where companies doing roadwork can check to locate any utility lines, which are marked, but no depth is given.

Pamatigan said his company consulted with all the utility companies before starting the repaving project and that none of them warned of shallow lines. Crews ran a metal detector before cutting, and there was no detection, he said.

Pamatigan said some of the pipes are plastic-coated and are harder to pick up with the metal detector.

Hawaii Gas spokesman Alan Tang said, "If you use a metal detector, you can detect location, but they need to check for depth. We don’t know what they were using."

The 4-inch steel pipe main was installed in 1977, so it’s hard to tell what has happened in the last few decades, Tang said.

Hawaii Gas on Monday night excavated three areas to shut off the gas, and in all three the lines were 2 to 3 feet deep, which is the recommended depth the company uses, he said.

Tang said Hawaii Gas responded quickly. The company was notified at 9:30 p.m. and dispatched two supervisors, one arriving quickly because he lived nearby. He met with the Fire Department, which had the situation under control, Tang said.

They set up a command center to assess the damage and develop a strategy to safely cut off the supply of gas. The first crew was on scene at 10:30 p.m. The remaining crews arrived by 11:15 p.m.

The gas pressure was first lowered. Workers located the three lines, excavated and created an area around the pipes, then pinched the pipes at three points to shut off the flow of gas.

Firefighters used multiple large-diameter fire hoses to surround the flames with a "curtain of water to prevent radiant heat from damaging other structures," Jenkins said.

HFD also notified the state Department of Health’s Hazard Evaluation and Emergency Response Office and the Coast Guard of the incident and the potential for 200 gallons of diesel fuel, 80 gallons of hydraulic oil and an undetermined amount of asphalt being released from the milling machine going down the storm drain.

Pamatigan said remedial and containment work was being done near the Ala Wai Golf Course due to the runoff and that the spill had been contained Tuesday, but the situation will continue to be monitored.

Pamatigan estimated damage to be about $700,000.

Jamba Juice cashier Ty Napeahi was closing up Monday night when the fire erupted. "People at Jack in the Box were freaking out because they’re right there."

Jack in the Box and Popeye’s and a handful of residential customers were without regular gas service Tuesday, Tang said.

The gas company provided Jack in the Box with a temporary gas tank and were working to run a temporary line. Popeye’s remained closed Tuesday.

Side Street Inn on Da Strip and another business also at 614 Kapahulu Ave. lost phone service due to the fire, Hawaiian Telcom said. Customers there and at nearby Jamba Juice could not use credit cards because the businesses had no phone or Internet service.