The search for a suspect in the killing of Baltimore Police Det. Sean Suiter entered a sixth day Tuesday — putting the case in uncharted territory for an agency with a recent track record of quickly apprehending killers of its own.

Since 1964, suspects in the killings of city officers have been caught within a few days, according to news accounts and a book, “Some Gave All,” which documents each line-of-duty death in the agency’s history. Even in the case of the three-month manhunt for the Veney brothers in 1964, the siblings were developed as suspects at the crime scene.

Suiter, an 18-year veteran, was shot around 4:30 p.m. Wednesday and died the following day. Police have said he was conducting a follow-up investigation on a triple homicide in the area, when he saw a suspicious person a vacant lot and approached. He was shot once in the head, the Baltimore Sun reports.