On June 28, 2021, about 3:35 p.m. local time, a natural gas explosion occurred during routine maintenance activities at an Atmos Energy Corporation (Atmos) facility near Farmersville, Texas.  The natural gas ignited and exploded after workers inserted an in-line tool (or pig) into a launcher.  Seven workers from Atmos, FESCO, Ltd. (FESCO), and Bobcat Contracting, L.L.C. (Bobcat) were on-site at the time of the accident and performing work for Atmos. The explosion occurred while the workers were removing the pig insertion tool during the sixth of a series of in-line inspections.  The explosion was directed through the open launcher door, ejecting the pig from the launcher, injuring two of the workers and killing two more. At the time of the accident, the weather was 75°F with light to heavy rain showers. No cloud-to-ground lightning strikes were reported within 15 miles of the accident site. Atmos estimated property damage at the site to be about $580,000.
The five surviving workers described the events leading up to the explosion in interviews with National Transportation Safety Board (NTSB) investigators. Shortly before the explosion, workers used a portable flaring system to vent natural gas from the launcher.  A worker opened the 1-inch valve that connected the flare flow line to the launcher and allowed natural gas to vent from the launcher to the flare tip, where it was successfully ignited. As natural gas pressure in the launcher decreased and less natural gas flowed to the flare tip, the work crew observed the flame die down and extinguish. The Atmos workers, who were overseeing the contractor personnel, did not expect gas to be leaking from the mainline valve, but they did note in interviews that they thought the flare system would provide a safe path for gas leaking past the mainline valve to vent to atmosphere if any leakage happened to occur.  The workers were not using any form of gas monitoring other than observing the flare tip. 
Workers began performing the remaining steps needed to load the pig about 3:28 p.m.: photographing the pig, opening the launcher door, lifting the pig into position with an excavator, attaching a grounding cable to the launcher and insertion tool, manually inserting the pig into the launcher, sliding the pig further into the launcher with the excavator, and removing the insertion tool. Before the insertion tool was completely removed, workers at the site heard a loud sound, and one worker observed a flash near the open launcher door.
An Atmos worker called 9-1-1 at 3:35 p.m. The Collin County Assistant Fire Marshal was the first emergency responder on the scene; he arrived at 3:41 p.m., determined that two of the four injured workers were deceased, and began to assist the two injured survivors. He observed no fire. Other responding agencies included the Farmersville Fire Department, the Texas Department of Public Safety, and the Collin County Sheriff’s Office. The Collin County Sheriff’s Office took control of the site and documented the accident scene. The Collin County Sheriff’s Office also called the Federal Bureau of Investigation to investigate for criminal activity. No criminal activity was found.
 (a) For more detailed information about this investigation, see the public docket at https://data.ntsb.gov/Docket/Forms/searchdocket and search for number PLD21FR002. Use the CAROL Query to search safety recommendations and investigations. (b) All times in this report are local time unless otherwise noted.
 A launcher is the portion of the pipeline facility used to insert in-line tools, commonly referred to as pigs, into a pipeline. Pig is a generic name for a device that is inserted into a pipeline and travels through the pipeline. There are numerous types of pigs used for inspection, cleaning, and separating products in pipelines. In this accident, the pig was a gauging pig for detecting dents or internal obstructions that could cause a pig to become stuck in a pipeline.
 Bobcat workers were providing pig loading services and FESCO workers were providing flaring services; Atmos personnel were overseeing the overall in-line inspections and approving each step in the process.
 An insertion tool is a device used to push a pig into a launcher. In this case, the insertion tool was a 16-foot-long metal pole with a cup welded to the end. (b) Pigs were successfully loaded on June 21, 22, 23, 24, and 27, 2021.
 A portable flaring system is used to direct natural gas from the launcher through a detachable flare flow line, up a portable stack, and to a flare tip, where it is ignited and burned.
 The mainline valve is one of two valves between the launcher and the transmission pipeline; the pig passes through the mainline valve to enter the transmission pipeline.
 The behavior of the flare tip would not indicate the presence or absence of natural gas, but it would provide information about the pressure of gas in the launcher. As such, it provided useful information but was not a reliable indicator of a safe or hazardous atmosphere at the site.
We determined that the probable cause of the June 28, 2021, explosion was a leaking mainline valve that allowed natural gas to enter the launcher where it mixed with air, creating a flammable gas–air mixture that was ignited by an undetermined source. Contributing to the explosion and its severity were Atmos Energy Corporation’s procedures and training practices that did not prepare workers to recognize and safely respond to abnormal operating conditions.