Police have seized cellphones they believe hold video of more than a dozen Edina High School students attacking a teenager in the city's YMCA and leaving him with brain injuries.

The fight erupted around 6 p.m. on Dec. 20 in the Southdale Y on York Avenue after 13 boys who attend the high school were allowed to sneak in through the back door, according to a police search warrant affidavit made public Thursday in Hennepin County District Court.

Once inside, the group gathered in the gym and assaulted the targeted teenager, the affidavit said.

The affidavit cleared the way for police to get court permission to search for video of the brawl on the cellphones of the 16-year-old victim as well as the phones of two others at the scene.

Police spokeswoman Lauren Siebenaler said Friday that no arrests have been made, and "the motive is still under investigation."

According to the affidavit:

No Y surveillance cameras captured the fight, but they did reveal there were individuals recording the incident with their phones. Police seized them at the scene ahead of extracting what evidence they might hold.

Paramedics arrived to take the victim to HCMC. He had a cut under one eye and numerous bruises.

"While in the back of the ambulance ... the victim's mental state was declining by the repetitiveness of his statements and lack of memories," read the affidavit, which police wrote on Monday ahead of its filing.