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Officials on Monday are trying to determine what caused hundreds of fish to die in the Malibu Lagoon recently.

Large groups of floating dead fish began showing up in the lagoon last Thursday, with much of the massive fish die-off still visible Monday morning.

California State Parks scientists are investigating the cause, but no official reason has been given.

One possibility officials are testing is whether higher than average water temperatures could be killing the fish, which appear to mostly be mullets, the Malibu Times reported.

Large die-offs have occurred in the past, but never to this level, California State Parks Angeles District Superintendent Craig Sap told the Times.

Some activists are blaming the die-off on a 2012 restoration project.

Resident Wendy Dunn told KTLA the State Parks agency ignored project management plans for proper breach points, which would allow the lagoon water to mix with fresh ocean water and keep oxygen levels where they need to be.

“When the citizens brought it their attention they called the citizens obstructionists,” Dunn said.

Officials have also been tasked with developing a plan to clean up the dead fish.

Crew members could be seen at the lagoon scooping dead fish out of the water.