Police in London are investigating a “minor explosion” on the city’s subway system during the middle of Friday morning rush hour.

The incident happened on the London Tube District Line at the Parsons Green station around 8:20 a.m., the BBC reported. BBC London presenter Riz Lateef, who was on her way to work, described the “panic” that resulted.”

Lateef said people “rushed from the train, hearing what appeared to be an explosion.”

The BBC employee recalled seeing people who had suffered cuts as they tried to get away.

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Richard Aylmer-Hall, a media technology consultant, said the affected train was “packed” at the time of the suspected explosion. Aylmer-Hall told the BBC that there was “lots of screaming.”

The BBC published an image of what it described as a white bucket in a paper bag. Twitter user @RRrigs posted the photo, adding that a “fireball flew down [the] carriage and we just jumped out open door.”

Aylmer-Hall said another woman recalled seeing “a bag, [and] a flash and a bang.”

Sophie Raworth, who was also on the train, said she saw emergency responders carrying a woman out on a stretcher. The victim appeared to have suffered burns to her face and legs.

British officials are currently investigating the reported explosion as a “security incident.”

Sky News reported at least 22 people are currently being treated for injuries. There were no immediate reports of fatalities.

UPDATE 5:40 a.m. ET: Counter-terrorism sources told BBC that they are treating the situation as a “terrorist incident.”

UPDATE 7:18 a.m. ET: CNN reported the device used in the explosion was an “improvised explosive device,” or IED.

UPDATE 7:35 a.m. ET: Sky News reported there was a timer attached to the device.