The birds were sent for necropsies at a federal forensics laboratory in Ashland, Ore. Officials ruled out natural causes, such as diseases from nearby poultry farms. The birds also showed no signs of trauma; they weren't shot.
Still, wildlife officials released few details about the deaths, saying they didn't want to compromise their investigation.
Bald eagles were removed from the endangered species list in 2007, but they're still protected under two federal laws, the Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act. Violators can be fined thousands of dollars and imprisoned.
In March, investigators turned their attention to prosecuting anyone responsible. It was Maryland's single largest die-off of bald eagles in at least three decades. But the six-month investigation and $25,000 reward failed to turn up those responsible.
Wildlife authorities and nonprofits initially offered $10,000 for information in the case. Soon the Center for Biological Diversity, an Arizona-based nonprofit, pledged another $15,000. An attorney for the nonprofit called the case "heartbreaking."
"We're saddened to see the case get closed," said the attorney, Catherine Kilduff. But the center's $15,000 reward still stands, she said. "We think this is a tragic loss for bald eagles and the people that care about them and their recovery."
The bald eagle was once at risk of extinction in the lower 48 states with fewer than 500 nesting pairs, according to the Fish and Wildlife Service. But the population has since recovered, and officials now estimate it at 143,000 eagles.
The 13 eagles were found dead on a farm on Laurel Grove Road in Federalsburg, just west of the 3,800-acre Idylwild Natural Area.
Then in March, five eagles were found dead about 30 miles east near Dagsboro, Del.
"In Delaware, our investigation is still ongoing, so we can't provide information at this time," Mendelsohn said in the statement.
Investigators believe the cases in Maryland and Delaware are not connected, said Terri Edwards, spokeswoman for the Fish and Wildlife Service.