When police told Jenny Neill about the human foot that had washed ashore on an island off Washington state, she had a sinking feeling: She knew it probably meant her son Antonio Neill, who vanished in 2016 when he was 22, was dead.

"The detective told me the size and make of the boot right when it was found, so I knew when it was a nine-and-a-half Timberland boot, [and] my suspicions were high," she tells PEOPLE.

Her worst fears were confirmed this week when the Snohomish County Medical Examiner announced that the foot, which was found on Everett's Jetty Island, belonged to Antonio.

"It has been a nightmare," she says.

Neill says she last saw her son on Dec. 12, 2016. He had been out of jail for just one day, and he said he had plans to stay overnight at a friend's house. But he never returned home.

The grieving mom suspects foul play.

"Somebody knows something," she says. "Somebody needs to come forward."