The U.S. Chemical Safety Board as well as the Occupational Safety and Health Administration are investigating a flash fire that injured seven contract workers, four critically, at Sunoco Logistics late last week.

Sunoco and the contractor, L-Con Inc. are each using their own third-party investigators to determine the cause of the Aug. 12 flash fire, according to information from Jeff Shields, communications manager for Sunoco Logistics Partners L.P.

Calls and an email regarding the medical status of the injured employees were sent to the communications department at L-Con but were not returned as of Monday afternoon.

The three contractors with minor injuries were treated at local hospitals and released. The others were brought to hospitals in Houston, Galveston and Beaumont for their injuries.

Workers were doing welding associated with connections to new crude oil tanks at the Nederland facility when the incident occurred, shortly before 9 p.m. Aug. 12.

There was no impact to nearby residents.

The Chemical Safety Board, headquartered in Washington D.C., is an independent federal agency charged with investigating industrial accidents, according to their website.