When 13 bald eagles were found dead on a farm in Maryland two years ago, the cause of death was a mystery.

Details of a six-month investigation, disclosed last week, show that the eagles died from ingesting a highly toxic pesticide banned in the United States, a spokeswoman for the United States Fish and Wildlife Service confirmed Thursday.

No arrests were ever made, and investigators have closed the case.

The bodies of the birds were discovered in February 2016 scattered on farmland in Federalsburg, Md., in the largest known die-off of bald eagles in the state in three decades.

The investigation results were first reported by the Annapolis radio station WNAV, which obtained them through a Freedom of Information Act request and shared them with The Washington Post.