State pollution inspectors are asking for help from farmers and other property owners to determine whether toxic runoff caused a massive die-off of brown trout in southeastern Minnesota last month on Rush Creek near Lewiston.

It was the area's third major fish kill in recent years, and past investigations have shown how difficult it can be to find an exact cause when any pollution quickly washes away downstream.

More than 2,500 fish were found dead July 25, many washed up on the creek shore. The Minnesota Pollution Control Agency believes the die-off was most likely caused by contaminated runoff following a heavy downpour. The rain could have carried freshly applied manure, farm pesticides or something else into the water. Investigators have talked to more than 100 landowners in the area, according a statement from the agency.

"We are asking landowners to provide detailed information about the type, rate, timing and location of manure and pesticide applications on their property," the statement read.

The state is also sampling insect life. If many small aquatic bugs were able to survive whatever killed the trout, it could suggest it was an intense but brief plume of pollution where some of the smaller critters were able to flee or burrow under the mud to wait it out.

Trout fishing and water quality advocates are concerned with how often fish kills have been happening in the area. Similar events in 2015 and 2019 killed thousands more fish in a river and a brook a few miles away from Lewiston. In both cases, the state wasn't able to determine the exact cause, but found that the kills were most likely the result of contaminated runoff.

The fear is that if the state once again can't find the exact site or sites where the pollution came from that nothing will be done to keep it from happening again, said John Lenczewski, executive director of Minnesota Trout Unlimited.

"If it turns out that it's just a few people who aren't informed or don't care, then yeah, we need to make sure they're not thumbing their nose at any rules or BMPs (best management practices)," he said. "But it seems like we have a systemic problem. So let's find out if our rules, BMPs, training, setbacks, inspections are adequate. Because if they're not, how are we going to keep this from happening again?"