The search for a man believed to have fallen into White Rock Creek is now a criminal investigation.
Initial reports indicated two men were fishing along the bank of the creek when one of the men fell in and disappeared. Officials now say that at 11:06 a.m. a fisherman spotted a man in distress in the creek and dove in to try to help the person, but that he was unable to reach the man before he went under.
Officials with Dallas Fire-Rescue searched for the man Monday morning, but were unable to locate him.
Officers talking with witnesses said "suspicious circumstances" surrounding the man's disappearance led them to call in homicide detectives.
Witnesses at the scene told detectives they heard "loud talking" coming from the direction where the man and another person were standing. A short time later, when the man was then seen struggling in the water, the fisherman dove in and attempted to rescue him, police said.
Police interviewed the person last seen with the missing man, who said he had just met the man and that he didn't know his name.
No further details about the interview were released by police and, so far, no arrests have been made.
A dive team with DFR will conduct a recovery operation in the water on Tuesday morning.
NBC 5's Ken Kalthoff contributed to this report.