LONDON – U.S low-cost carrier, Southwest Airlines, has become the latest company to be subpoenaed for its Boeing 737 MAX documentation.

Other companies, including pilot unions from American Airlines (AA) and Airlines (UA), have all also been given federal grand jury subpoenas for documentation related to the grounded Boeing 737 MAX.

Southwest has the largest Boeing 737 MAX fleet with over 34 planes currently grounded, and an additional backlog of more than 200 planes on order.

The Airlines Pilot Association (APA) was given demands from the US Department of Justices’ criminal division to comply with the request by May 24, according to union president Jon Weaks.

Southwest 737 MAX 8. Boeing 10,000th 737 Guinness World Record. PHOTO: Brandon Farris.

In an interview last week, Weaks said, “I don’t know what aspect they are investigating, they just want to know what we have on the MAX. We knew it would come eventually.”

It is understood that the Southwest pilot’s union is looking for an extension on the deadline due to the time it will take them to collect all its information and emails regarding documents linked to their 737 MAX aircraft.

“Southwest is cooperating fully with the request,” said company spokeswoman Brandy King.

In a report from the Business Times, it is reported that a source has informed that the Allied Pilots Association, whose member work for American Airlines, has also received one subpoena.

RENTON, WA – JANUARY 29: A Boeing 737 MAX 8 airliner lifts off for its first flight on January 29, 2016 in Renton, Washington. The 737 MAX is the newest of Boeing’s most popular airliner featuring more fuel efficient engines and redesigned wings. (Photo by Stephen Brashear/Getty Images)

The investigation for Boeing’s newest single-aisle jet has been ongoing ever since the 737 MAX was grounded in mid-march.

However, an investigation into the MCAS system onboard the MAX had begun just a few weeks after the Lion Air crash.

The investigation is still ongoing, however. In both incidents, the new software system was reported to have repeatedly shoved the nose of the doomed jets in a downwards motion.

Recent reports have been released, which outlined questions in relation to the aircraft being certified for commercial operations with the current MCAS software onboard.

While no information has been released on the reasons for the subpoenas, it is believed that the main focus of the investigation is to now identify how the MCAS system was certified by the FAA and why Boeing did not flag the new software in their pilot Manuals.