OKLAHOMA CITY (AP) — An explosion and fire at a southeastern Oklahoma natural gas well that killed five workers followed the removal of a pipe from the well but an ignition source still hasn't been identified, federal investigators said Thursday.

Officials from the U.S. Chemical Safety and Hazard Investigation Board said they are conducting tests on the equipment involved — including a blowout preventer that failed to close — as they updated the status of the investigation into the Jan. 22 blast near Quinton, about 100 miles (160 kilometers) southeast of Tulsa.

"This is a complex investigation...