ASHLAND, Ore. - Investigators looking into the death of thousands of fish in a tributary of the Rogue River have a new clue: An irrigation district says it accidentally flushed herbicide into the creek.

But state officials say they may never pinpoint what killed the fish on Bear Creek because of their delay in collecting soil and fish samples.

Investigators estimate more than 3,000 steelhead smolts, 114 young coho salmon, 19 rainbow trout and thousands of nongame fish were killed in a five-mile stretch between Ashland and Talent.

A gate on a drainage ditch was mistakenly left open May 8 as Talent Irrigation District workers sprayed Acrolein, an aquatic weed-killer, on algae and moss clogging the district's 20-mile canal.

"I'm not saying that's what killed the fish, but the timing is right," district manager Hollie Cannon said.

Tests have shown Acrolein can kill chinook salmon at a concentration of 0.08 milligrams per liter, according to the Ashland environmental group Headwaters.

State environmental tests of water and sediment from the creek Friday showed no detectable amounts of any toxin. Test results from fish tissue samples were expected this week.

"We are very concerned that we've got a smoking gun, but because of gross mishandling on the part of state agencies . . . it'll be too late to determine what caused this," said Gary Schrodt, a member of the Ashland Wetlands Coalition.

Department of Fish and Wildlife workers counted dead fish May 9 but did not collect a fish sample. An official with the Rogue Valley Council of Governments collected nine dead fish the next day after getting a call from the state Department of Environmental Quality. The fish sat in a freezer for six days until being delivered to the Fish and Wildlife department. A private citizen took soil samples for DEQ two days after the kill was discovered.

Fish and Wildlife biologist Mike Evenson said freezing fish can trap chemical traces, but it also can do the opposite.