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By Andrew Rudansky, Elizabeth Chuck and Cassandra Vinograd

Six teens were injured when an explosive device was thrown into a backyard party in northern California, police said early Saturday.

The Manteca Police Department said officers responded to reports of numerous injuries from an explosion at a local home shortly before midnight.

"Preliminarily, we're thinking that it's a modified firework that was thrown into the backyard," Manteca police Chief Nick Obligacion told NBC News.

Around 60 high schoolers were attending a party at the home when "an unknown subject intentionally threw an explosive device over a fence" into the backyard, according to a statement from police.

There were six victims, Obligacion said. Four were treated and released with minor to moderate cuts and burns, one was transferred to a county hospital with moderate injuries and another was transported to Oakland Children's Hospital for a "more severe facial injury that doesn't require surgery," Obligacion said.

All of the victims were juveniles, he added.