Madison authorities continue to investigate whether a murder suspect was responsible for two other killings. But the suspect's father says his son couldn't have committed the other two crimes.

20-year-old Adam Peterson was charged last week in the death of Madison resident Joel Marino in January.

That led police to wonder whether Peterson was linked to two other unsolved murders. UW-Madison student Brittany Zimmermann was killed in April, and 22-year-old Kelly Nolan was killed last summer.

Melvin Peterson tells the Minneapolis Star Tribune his son was in Minnesota when Zimmermann died, and in Wisconsin Dells the night Nolan disappeared.

Authorities said Friday they had no forensic evidence linking Adam Peterson to other homicides but would continue to investigate.