LIBERAL, Kan. (AP) — Authorities are trying to determine what caused an explosion and fire at a southwest Kansas business.

The explosion Tuesday at Groendyke Transport Inc., southeast of Liberal, injured four people.

Seward County Fire Chief Andrew Barkley says a propane or natural gas tanker inside a large metal building exploded and caught fire for still-undetermined reasons.

The Hutchinson News reports fire crews believe the tanker was empty but employees reported gas vapors were released.

Three employees were first taken to a Liberal hospital and then flown to a Wichita hospital. They were in critical but stable condition. A fourth employee was treated for minor injuries.

U.S. 83 was closed south of U.S. 54 in Liberal for about 2½ hours. No homes were endangered because the company is in a rural area.