The MTA and NYPD are investigating an explosion on the C line apparently caused by a piece of metal put onto the track.

A 20-second video posted to YouTube on Saturday of a C train rolling into an empty Nostrand Avenue station shows a bright white flash and sparks flying over the sound of a loud boom.

The incident happened Thursday morning at 8:14 a.m. on the Queens-bound tracks of the C line, according to MTA spokesman Adam Lisberg. The train operator hit the emergency brake and eventually turned the power off, with service resuming at 9:20 a.m. Metal debris was found in the track but not yet confirmed whether it was a piece of equipment.

The person who uploaded the video, which Gothamist picked up Sunday, "was not involved. I was not present at the time," the caption reads.

"Vandalizing transit property is illegal and unsafe. The people who did this need to re-evaluate their conscience," wrote the person who uploaded it under the name Dj Hammers.

That person, Max, 18, who asked to withhold his last name, told amNewYork he ripped the video from Facebook before it was removed. He listed five names of people he suspected of the dangerous prank; the names were later removed.

He documented a history of mischief -- posted to social media -- in the "Railfan" community of subway and train enthusiasts.

"Over the course of a year, nothing's been done," Max said. "The thought was, getting it out in public on a fairly widely watched YouTube channel would finally put an end to this. It negatively impacts commuters and the workers."

Authorities are looking into the social media postings of a group of teens as the culprits, according to Lisberg. However, officials could not verify that these specific names are being investigated by the NYPD.

"If this video really depicts what it claims, it is unconscionable that vandals would have deliberately endangered the lives of our customers and employees, as well as delayed thousands of customers during the morning rush," the MTA said in a statement. "We will work with the NYPD to identify and capture the perpetrators responsible for this."