competition: Most confusing RIDDLE

There were two investigators investigating on a suicide, a guy was left dead on the floor holding a gun on his left hand and a recorder on his right. the 1st investigator pressed play on the recorder and it said "i am commiting suicide--(then gunshot)". the 2nd investigator said, "this isn't a suicide, this is a murder".

Q: How did the investigator know that it was a murder?