The 19-year-old Jupiter man accused of fatally stabbing two Martin County residents last week and seriously injuring another remained in critical but stable condition Sunday, the Martin County Sheriff's Office said.

Austin Harrouff has been at St. Mary's Medical Center since he was taken into custody shortly after the deaths of John Stevens and Michelle Mishcon late Monday at their home just north of the Jupiter border.

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The Sheriff’s Office plans to charge Harrouff, an Florida State University student, with two counts of first-degree murder and one of attempted murder once he is released from the hospital. Doctors have said that release could come as soon as this week.

The FBI is running tests to see whether Harrouff was under the influence of either bath salts or flakka — which have been linked to violent outbursts — at the time of the attacks. Tests done last week found no traces of street drugs such as marijuana, methamphetamine or cocaine.

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Harrouff had told his mother that he had super powers and was immortal, said a Jupiter police report released last week. Worried about his behavior, she had police and his fraternity brothers search for her son after he grew upset and walked out of Duffy's Sports Grill in Jupiter shortly before the attacks.

Investigators have not been able to take a statement from Harrouff, who is medicated and breathing with the help of a tube.