GRAND JUNCTION, Colo. (KKCO) -- Investigators are still trying to determine what caused a natural gas leak that caught fire and blew up a house.

A release from the Mesa County Joint Information Center says right now, officials know a private company contracted by the City of Grand Junction was working to upgrade a traffic signal at 7th Street and Walnut Avenue on Tuesday.

Workers were boring into the ground, and at some point, a natural gas line was hit, the release says. That line provided gas to Colorado Mesa University.

That damage caused a leak of about 40 pounds per square inch of gas, which escaped into the sewer system, according to Grand Junction Police Chief John Camper.

The leak made its way into the house at 1752 7th Street, and at 12:50 p.m., it caught fire and exploded, leveling the house and burning the home next door to the ground.

Three people were injured in the explosion. The two who were inside are still being treated at St. Mary's Hospital and are listed in serious condition as of 1 a.m. Wednesday. The third, who was just outside the house that blew up, was released from the hospital Tuesday evening.

Xcel Energy crews are now repairing the line, but the fix is taking longer than expected because the break is seven feet underground. Meanwhile, other workers are going house to house in the area, checking gas levels to see if residents will be allowed to return home.