Four NYPD detectives have testified before a federal grand jury probing leaks of top-secret information about a terror plot to blow up city subways, a source familiar with the investigation told The Post.

Two testified before the grand jury in Washington last Friday and two more testified yesterday.

The detectives were assigned to the FBI-NYPD joint terrorism task force, the source said.

As many as nine NYPD officers have received subpoenas as part of the probe, the source added.

The investigation is focused largely on the Najibullah Zazi case and leaks of sensitive information from the task force to cops without clearance.

(Source: NY Post)