WESTMINSTER, Colo. (CBS4)– Hundreds of dead fish have washed up in Westminster City Lake Park since last week. The 4th of July Fishing Derby has been cancelled because the mystery behind what’s killing the fish remains unsolved.

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Neighbors told CBS4 that dozens of dead fish wash up on the shores every day. Each night, crews remove the fish but in the morning there are more.

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“Something really funny is going on,” one neighbor named Theresa said.

Theresa loves to walk around City Park Lake in Westminster.

“It’s so pretty. There’s lots of different types of birds,” she said.

Westminster Fire tweeted the upcoming 4th of July Fishing Derby has been cancelled because of the dead fish.

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The city is working with an outside company that manages the pond to find out what went wrong.

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The lake was stocked with thousands of fish; carp, catfish, bluegill and large mouth bass. Now all of those fish are dying.

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“We think at this point that most of the fish are gone,” Jodie Carroll, a spokeswoman for the City of Westminster said.

They haven’t gotten the results of the investigation back just yet. For now, all they can do is just keep cleaning up the fish.

Some people who live in the neighborhood are concerned about the other animals that call the lake home.

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“Well, I think it’s sad because there’s a lot of wildlife out here and I’m afraid the wildlife are going to ingest the fish and then there will be more dead wildlife,” said Theresa.

The city says no other wildlife has died so far. They should be getting the results of their investigation into the die off by the end of this week or beginning of next week.

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They also say that if people want to participate in a fishing event, they are having another one in August at a different location.