One of 13 bald eagles found dead in a farm field on Maryland's Eastern Shore earlier this year. (Maryland Natural Resources Police )

They had the carcasses of the 13 bald eagles discovered on Maryland’s Eastern Shore. But federal wildlife experts say finding enough evidence to identify the culprit in the largest single die-off of bald eagles in the state in 30 years has so far proved too difficult for the mystery to be solved.

The case of the dead eagles found six months ago in a field in Federalsburg drew national attention, and investigators from the U.S. Fish and Wildlife Service have been involved ever since in trying to figure out whodunit.

The birds’ carcasses were sent to a lab in Oregon and analyzed. Tips came in, and a $25,000 reward was offered. But authorities now say they do not have enough evidence to prosecute anyone.

“None of the leads have been fruitful,” Terri Edwards, a Fish and Wildlife spokeswoman, said Monday. “We haven’t had any success with the leads.”

The dead bald eagles in Maryland were sent for analysis to the U.S. Fish and Wildlife Forensics Laboratory in Ashland, Ore. (U.S. Fish and Wildlife Forensics Laboratory)

Many eagle experts speculated that the birds might have come into contact with a pesticide at a local farm. And federal investigators said that they received several leads and put in “considerable effort” in chasing them but that nothing turned up.

Officials with the Fish and Wildlife Service would not detail this week any of the findings in their investigation. They also said they were not sure when the case would officially close.

Neil Mendelsohn, a regional assistant special agent in charge at Fish and Wildlife, said in a statement that “although we conducted a very thorough investigation into the Maryland eagle poisonings, we are intending to close the case in the near future due to a lack of evidence linking anyone to the crime.”

The federal agency was also investigating a case involving five dead bald eagles found in late March in Sussex County in Delaware. Mendelsohn said that investigation is ongoing, but said that “we can’t provide additional information at this time.” Those eagles were also examined at the lab in Oregon.

Bald-eagle enthusiasts said they were “disappointed and frustrated” that the Maryland and Delaware incidents had happened at all but not entirely surprised by the outcomes.

Ed Clark, president and founder of the Wildlife Center of Virginia, said he is confident federal investigators “diligently tried to identify the culprits” and “gave it their best” in the case. He said it is likely someone has stored an insecticide that is now banned from the market and used it. But tracking records of who might have bought such banned products is tough, experts said.

Someone “sprinkles this stuff,” Clark said, “and it becomes a toxic time bomb.”

In the Maryland case, the dead eagles were found Feb. 20 in Federalsburg by a man who said he was out looking for antlers that deer might have shed.

The man came across what he initially thought was a dead turkey in a field on a farm on Laurel Grove Road before discovering it was the remains of four dead eagles. When wildlife authorities arrived, they found nine more nearby.

At the time, officials said, it was not immediately clear what had caused the birds to die and there were “no obvious signs of trauma,” Maryland Natural Resources Police said.

Experts on bald eagles have said they still think the birds’ death involved some sort of pesticide or poison that may have been used to get rid of predators of livestock or rodents. If the animals died outdoors and the eagles ate their carcasses, the birds also could have gotten sick.

At least three of the 13 birds that were found dead were mature, with the signature white heads and brown bodies. Two of the birds were close to being mature, officials said, and the rest were considered immature birds with no white feathers.

Their bodies were sent to the Fish and Wildlife Service Forensics Laboratory in Ashland, Ore., basically a CSI lab for animals, where they underwent a thorough analysis. Veterinary pathologists examined the carcasses. X-ray images were taken, and samples of liver, stomach, kidney, urine and blood were collected and analyzed.

In March, officials said a necropsy showed the birds did not die from natural causes or disease. Investigators repeatedly said they were looking for suspects in their deaths. And a $25,000 reward was offered for information that leads to an arrest and conviction.

Bald eagles were once on the endangered-species list but were removed from it in 2007. The birds remain protected under several federal laws and are recognized as the national bird.

Two years ago in a high-profile case in Wisconsin, farmers were fined and ordered to pay restitution after using illegal pesticide to try to get rid of coyotes and wolves. More than 70 wild animals, including two bald eagles, vultures, coyotes, owls and a bobcat were killed.

Clark said it is upsetting that the bald eagles were killed, given that they were almost extinct and have now come back as the result of conservation efforts. Federal wildlife officials said there are now roughly 143,000 individual bald eagles in the U.S.

“To think after all that effort to bring bald eagles back and we still have people willing to squander it,” he said. “These are public treasures. Someone has vandalized, stolen and destroyed from the public these wonderful birds.”