ISSAQUAH, Wash. -- It all began when a biologist smelled trouble while surveying spawning salmon in Issaquah Creek.

The biologist noticed some 40 dead fish along a quarter-mile stretch of the creek. They were of multiple species -- sculpin, trout and Chinook salmon.

Then the scientist at the state Department of Fish and Wildlife noticed the stench of ammonia in the air. That stench led him to a nearby plant.

According to the Department of Ecology, an ammonia leak from a nearby processing plant is to blame for the dead fish.

Darigold's processing plant, which uses ammonia for its refrigeration system, leaked the chemical from its roof, investigators said.

The leaked ammonia then mixed in storm water, shot out through a pipe and right into the east fork of Issaquah Creek.

"They knew there was a leak (Wednesday) morning and shut down their system to fix it. They did not know it went to surface water," said Lori LeVander with the Department of Ecology.

After a day of pH-level testing, biologists declared the creek water safe again on Wednesday night. And to be sure it stays safe, Darigold employees will check the pH levels of the water leaving the plant every two hours.

"We are doing everything as a community to protect salmon and water quality. And (when) something like this happens, it can wipe out a stretch...(it) can have some serious impacts," LeVander said.

"It's hard to see fish die. Healthy fish coming back to spawn after all they've gone through to go out to the ocean and come back, then to die instantly because of a spill."

The Department of Ecology, along with the Environmental Protection Agency and the Department of Fish and Wildlife, plan to launch an investigation with collected fish tissue samples.