An Indian airline accidentally left 55 passengers behind after staff forgot they were still on one of four buses that were transporting customers to the aircraft. India’s civil aviation regulator has reportedly demanded an explanation from the airline over Monday’s snafu.

Go First flight G8-116 was already running around 30 minutes late on Monday when airline staff decided to shut the aircraft doors and get the flight underway despite the fact that 55 passengers hadn’t yet boarded the plane.

The Bengaluru to Delhi flight ended up arriving just 13 minutes late, although the 55 forgotten passengers didn’t arrive in Delhi until four hours later after they were rebooked on a later flight once staff became aware of what had happened.

Responding to passenger complaints on Twitter, the airline said: “We regret the inconvenience caused.”

It remains unclear why the bus didn’t make it to the aircraft in time or why the airline didn’t wait for the bus to arrive. The airline says it will investigate the incident but has declined to comment any further on the matter.