TOLEDO, Ohio — Investigators have linked three large fish kills this month to livestock manure that had been spread on farm fields in northwestern Ohio, and now they’re looking into whether any laws were broken.

What’s not known is whether the manure was deposited just before rain washed it into area creeks or if too much manure was put down, said Jeremy Payne, a wildlife investigator with the Ohio Department of Natural Resources.

An Ohio law put in place two years ago to combat harmful algae in Lake Erie prohibits farmers from spreading manure on fields within 24 hours of expected heavy rains because it contains phosphorous that feeds algae.

Ohio, along with Michigan and the Canadian province of Ontario, have agreed to sharply reduce the amount of phosphorus from farm fertilizers, livestock manure and sewage treatment plants that flows into western Lake Erie within the next 10 years.

But many environmental groups argue that the state needs stricter rules on disposing manure and that voluntary efforts are not enough.

The manure also contains ammonia that pulls the oxygen out of creeks and streams and kills fish. An estimated 66,000 fish — from minnows to sunfish — were found dead in the creeks in Hardin, Allen and Williams counties this month.

A fourth fish kill in Mercer County also is being investigated.

In the most recent case, about 15,000 fish along a 10-mile stretch of a creek in Williams County were found dead last week.

The manure spill, though, won’t cause any long-term damage in the creek, said Payne, who added fish already are being spotted in the Williams County creek.

It’s possible that those responsible for putting the manure on the fields will be billed for the cost of the fish, said Steve Thomson, a state wildlife officer.

How much depends on the type and number of fish killed, but the price tag could be several thousand dollars, he said.