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Authorities arrested an 18-year-old southern Illinois man who they allege had communicated with an unspecified terrorist group about a plan to an attack on at least one area location.

Keaun L. Cook, of Godfrey, was arrested Wednesday on preliminary charges of providing material support for terrorism and making a terrorist threat. He was being held at the Madison County jail on $150,000 bond and didn't have a lawyer as of Friday morning.

At a news conference Thursday, Madison County's state's attorney, Tom Gibbons, described Cook as a dangerous man who had deliberate plan to cause a "mass casualty event" at one or more locations in the county. He declined to specify which terrorist group Cook had allegedly been in contact with, but said they weren't local. He also wouldn't give the locations of any planned attacks, but said police departments and individuals at those locations were notified after authorities first learned of the threat.

County Sheriff John Lakin said authorities learned of the threat Aug. 24 when deputies did a welfare check at the home of Cook's grandmother, who has looked after him since his mother died unexpectedly of an illness in 2011. Gibbons said someone then reported the verbal threats.

Although investigators found no dangerous materials or firearms in the home, Lakin said he thinks there was a "strong possibility he could have carried it out alone."

"I'm very proud to stand here today and say that we stopped an event that could have caused a very, very, very serious situation," Lakin said.

Cook's grandmother, Debra Thomas, hand-delivered two letters to The (Alton) Telegraph (http://bit.ly/2c7oq93 ) on Thursday. In them, she wrote that her grandson struggles with paranoid schizophrenia and that he had spent more than 300 days in isolated confinement at a county detention center, during which his condition went untreated.

She wrote that after he got out and returned home, she couldn't force him to take his medication because he is 18 and an adult. She said she called the police, "not because that I felt a threat but because I knew that was the only way that I could get Keaun treatment."

"When he's on his medicine he is the sweetest person you know, when off like Dr. Jekyll and Mr. Hyde," Thomas wrote, noting that she never would have involved the police if she thought it would have landed her grandson in jail.

Thomas didn't immediately respond to a phone message from The Associated Press seeking comment.

Gibbons' office said further details about the alleged plot wouldn't be released yet because the investigation is ongoing.

Cook has no previous felony convictions in Madison County. He did have one 2011 misdemeanor conviction for damage to property for using a rock to scratch a pickup truck. Cook has no charges in St. Clair County or in nearby Missouri.