LINCOLN, Neb. (AP) — A team of fire investigators is trying to determine the cause of a natural gas explosion in Lincoln that damaged nearly 20 homes and gave two people life-threatening injuries.

Investigators and Lincoln police spent a second-day searching rubble for reasons behind Monday’s home explosion. Investigators didn’t find any evidence of a gas leak outside the home Tuesday, and Black Hills Energy officials reported no issues with its service lines to the home.

Chief Fire Investigator Bill Moody says investigators will try to determine if a mechanical failure, accident or foul play triggered the blast.

Fire officials say the explosion threw homeowners, Jim and Jeanne Jasa. A hospital spokesman says the couple remains in critical condition as of Wednesday.