Call it CSI: Middle River.

State officials and local residents are investigating why more than 100,000 fish turned up dead in this Eastern Baltimore County tributary that, in recent years, has been known for fine fishing and celebrated bass tournaments. They are coming to the pier along Wilson Point Park, looking at the dead pickerel and yellow perch and crappie, and sharing information about what set off a bloom of toxic algae that killed the fish.

The Maryland Department of the Environment confirmed that a toxic algae, Karlodinium veneficum, was the culprit. Karlodinium is a type of harmful algae, a dinoflagellate, that more typically blooms in the summer when the water temperature is more than 60 degrees. It blooms throughout the Chesapeake, a common scourge fueled by an excess of nitrogen and phosphorus. But why did it bloom here, and now?

An unseasonably warm early November might have brought on the bloom by letting the algae survive longer and thrive. But, some wonder, was it hot enough? After all, 60 degrees in November is not unprecedented in Baltimore, and previous seasons’ warm days didn’t trigger blooms in Middle River.  Even if it was warm enough, the temperatures didn’t bring the algae to the creek.  As biologist Chris Luckett put it, the grenades were already in the creek; the warm weather just helped pull the pins.

Maryland Department of the Environment biologist Charlie Poukish said he’s seen kills like the Middle River one often enough to identify the cause - algae - and the root of it, nitrogen and phosphorus that fuels the algae bloom. Yet Poukish was as uncertain as anyone else as to the source of that nitrogen and phosphorus. The pollutants generally come from stormwater discharges, sewage treatment plant overflows, and agricultural runoff.

He’d heard about a stormwater discharge in the area. He’d also been fielding calls from anglers who believed the fish kill was the result of an illicit discharge from a chemical company. Others believed a discharge could be coming from Martin State Airport and the former home of Martin Marietta, which made aircraft there for half a century. The facility is involved in a road resurfacing project as well as a major cleanup of contaminants in its soil, including PCBs.

Allen Blevins, a longtime angler who used to work for Martin Marietta, was skeptical that the heat fueled the algae bloom at a cool time of the year. If it were stormwater, he said, it would be more widespread, instead of just a few creeks. Something clearly happened near Wilson Point Road. But what? Blevins wasn’t sure.

“We could come down here and catch 50 yellow perch within two hours, and then, all of a sudden, it was like you flipped a light switch,” Blevins said. “For anglers like me, it’s devastating. We were having the best fall fishing in 10 years. It was like fishing in the Florida Keys.”

This week, with the sun out and 60-degree temperatures, it was easy to imagine oneself, for a moment, in a Beach Boys song. The bottom of Cow Pen Creek was gin-clear. Aquatic plants swayed underwater. But at their base, more often than not, lay a dead menhaden, pickerel, bass or bluegill. The carnage stretched from Norman to Hopkins to Dark Head Creeks, in the Hawthorne area of Middle River.

“I’ve been crying for three days, man,” said Scott Aubrey, a longtime fisherman and activist who has complained in the past that the state has not done enough to combat pollution from industry in the area.

The Middle River fish kill was the largest this year, and it comes at a time when anglers and environmentalists have taken great care to help the tributary rebound. Once riddled with pollution from nearby industry, the Middle River fishery has bounced back in recent years. In August, angler Aaron Martens won the Bassmaster Elite Series with a fish he caught in Middle River. Local anglers, including Alewife chef Chad Wells and fishing guide Scott Sewell, have been stocking the river with juvenile fish and helping to restore habitat for several years.

As the fishery improved, the Wilson Point Civic Improvement Association worked with the Gunpowder Conservancy to secure grants for homeowners to address stormwater. To date, they have helped install 156 rain barrels and 37 bay-friendly gardens, said Dan Doerfer, who is active on the association’s board.

“I was so proud that this was my fishery,” Wells said in a post on Facebook. “I see so many people posting pictures of the fish they catch with their kids and friends here and it's always my favorite thing to see. Now we have this. All of this work and effort put in by so many squandered.”

On the water this week, Wells was downcast. The kill, he said, had hit his creek just the day before. Explaining all the dead fish to his 3-year-old, he said, was not something he enjoyed - especially as father and son watched the birds swoop in for a feeding and Wells worried they, too, could fall ill.

Sewell, who is conservation director of the sportfishing group Maryland Bass Nation, has been posting video of the fish kill on social media and calling his state representatives to come out and see the devastation. Sen. John Salling, of Dundalk, came out Monday and promised action.

“We need to find out what happened here. We need to make sure it doesn’t happen again,” Salling said. “You see death all around here, and it’s so sad.”

Sewell began to think more and more that the fish kill came from a discharge, so he got Poukish and Luckett to investigate. State biologists have been to Middle River several times this week, but Sewell wanted to make sure they saw a discharge pipe in Cow Pen Creek. Poukish pulled on his waders and took a sample, but said whatever was coming out was just a trickle and was unlikely to cause such a widespread kill. Plus, Luckett helpfully noted, the first report of the fish kill was not close to the pipe.

Near the discharge pipe, the water smelled like burnt rubber. But the sample Poukish collected smelled fermented, and was difficult to identify. What would the lab test it for? Poukish wasn’t sure, but Sewell had a list of the chemicals manufactured at a nearby company, and that could help them get a start. Sewell had also taken his own samples near the pipe and in the water.

Luckett and Poukish discussed the various causes, but they reiterated they just didn’t know. Maryland fish have suffered from 80 smaller fish kills this year; in some cases, the culprit is never specified other than algae from excess nutrients.

They’re hoping that, in Middle River, they’ll come up with a more specific answer.

“We’ve collected a lot of evidence,” Poukish said. “It’s just a matter of sorting through it now to see if it makes sense.”