A Baltimore homicide detective was fatally shot in the head with his own gun after a struggle with his killer last week, the city’s police commissioner said Wednesday.

Sean Suiter, 43, was to testify before a grand jury the day after he was killed, Commissioner Kevin Davis said. The grand jury is looking at a corruption case involving several Baltimore officers who were federally indicted in March.

Suiter, an 18-year department veteran, was investigating a killing in west Baltimore on November 15 when he saw a man exhibiting suspicious behaviors, police said. The officer died the next day at a hospital.