St. Paul police say they’re investigating a homicide in the city’s Union Park neighborhood.

According to the St. Paul Police Department, one person was shot and killed Monday in an apartment building on Marshall Avenue, just off Prior Avenue.

When officers got inside the building, they found a man in a unit with a gunshot wound to the head, as well as a gun.

Police say the call first came in as a suicide around 2 p.m., but after speaking with witnesses, their investigation switched to a homicide. Detectives said the circumstances weren’t adding up.

“In this case, they spoke to witnesses involved and did discover inconsistencies in the witness statements. Based on those inconsistent statements, our investigators did find probable cause to arrest an 18-year-old,” said St. Paul Police Department Sgt. Mike Ernster.

Police identified that 18-year-old as a man, adding he is believed to be the only suspect in the case and the public isn’t in any danger. He is being held on suspicion of second-degree murder.

The man’s death is the sixth homicide within the city this year.

The names of the suspect and victim haven’t been released as of this time but are expected to be announced later this week.