Investigators say they have few clues as to the whereabouts of missing Florida construction company owner Susan Fast, who has been missing for three days.

And the more time that goes by, the more authorities are worry whether they will find her alive.

"As hours and days go by, it gets less hopeful," Dave Bristow, spokesman for the Manatee County Sheriff's Office, told the Bradenton Herald Monday. "But there's always hope."

Fast's stepson, Thomas Fast, who was arrested recently on a separate concealed weapons charge, is considered a person of interest by police because he was the last person believed to have seen her, reports FOX affiliate WTVT.

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His criminal history includes convictions for petit theft, worthless checks and prostitution-related charges. His bail was raised to $100,000 on Sunday morning.

"He admits that he was there at the house and spoke with her at 5 p.m. ... so that's essentially the last person that may have seen her, and so we do suspect foul play because she hasn't made any contact," Manatee County Sheriff's Office spokesperson Major Connie Shingledecker told My FOX Tampa Bay.

Susan Fast's husband, Bruce, reported her missing after returning from a business trip. The couple runs the construction company together.

"He arrived home later that evening he couldn't find her, he couldn't find her vehicle and that she was not answering her cell phone," Shingledecker said.

At a bail hearing Sunday, a detective said more than a dozen spots of blood were found in the Fasts' home and a set of steak knives were found clean in the dishwasher. Bruce Fast said his wife normally didn't wash the knives in the dishwasher, the Sarasota Herald-Tribune reported. Also, two rugs were recently washed and a fresh cut was on Thomas Fast's elbow, the paper reported.

Detectives found her Lexus SUV about 10 miles away from her home with keys still in the ignition and a missing tag.

The Bradenton Herald reports that relatives of a 16-year-old boy who lives directly across the street from where the vehicle was found at 603 11th Ave. said the teenager told investigators that he watched a man in his 40s get out of a Lexus at about 3 a.m. Saturday.

"He was a white guy in his 40s, he held two big old black bags and walked north on Sixth," one of the teen's relatives told the Herald.

Click here to read The Bradenton Herald story