(Reuters) – Five bald eagles have died in Delaware, state officials said on Tuesday, weeks after 13 of the U.S. national birds were determined to have been killed by humans in neighboring Maryland.

The Delaware Department of Natural Resources and Environmental Control said it was investigating what killed the eagles, but would not comment publicly on possible causes.

Three of the eagles were still alive and very ill when they were discovered at the weekend, Sgt. John McDerby of the Delaware Fish and Wildlife Natural Resources said in a statement. They died a short time after their rescue.

“We don’t know how many eagles may have been affected, so we are asking the public to notify us immediately should they see birds that appear sick,” McDerby said.

Thirteen bald eagles were discovered dead in Maryland last month, with lab results indicating the birds did not die of natural causes, including diseases such as avian influenza, according to the U.S. Fish and Wildlife Service.

The agency declined to say whether the birds were poisoned but said the investigation was focusing on humans as the cause of death.

The 13 birds represented Maryland’s largest bald eagle die-off in 30 years, officials said.

The bald eagle, which almost disappeared from the United States decades ago, was removed from the federal endangered species list in 2007 after habitat protection and the banning of the pesticide DDT led to its recovery.

The federally protected bird is a symbol of the U.S. government and is featured on currency and in the presidential seal.

The maximum fine for harming a bald eagle is $100,000 and up to one year in prison.

(Reporting by Victoria Cavaliere in New York; Editing by Paul Tait)