High pressures in a stretch of piping likely caused an explosion that knocked the Freeport liquefied natural gas (LNG) export terminal offline, according to federal regulators, who have proposed requirements that could ultimately delay a return to service.

The Pipeline and Hazardous Materials Safety Administration (PHMSA) said the cause of the June 8 incident is still under investigation. However, preliminary evidence suggests an isolated pressure safety valve created an “overpressure situation” in 300 feet of vacuum insulated piping.

“The 300 feet of pipe was subjected to an overpressure situation which burst the pipe and allowed LNG and methane to be released into the facility,” PHMSA said Thursday. The agency issued a notice of proposed safety order Thursday that lays...