Over 1,900 police detectives lost their qualification this year to lead investigations after refusing field assignments too long, the National Police Agency said Monday.

Detectives are reviewed yearly for renewal of their qualifications under a system introduced in 2005. This year’s number of investigators losing their status is the highest since the system was introduced, the NPA said.

The total number of qualified special investigative officers now stands at 31,659, down from 33,615 last year.

“We will replace those not willing to investigate with new personnel and manage the pool more efficiently,” an NPA official said.

To specialize as an investigative officer, police officers should pass a written exam, held every June, or earn a recommendation from the head of an office.