NYC Police Threatened 40 Times Since 2 Cops Killed

About half still under investigation, with four arrests.

Half of those threats have been deemed not credible. The rest are still under investigation. There have been four arrests in the four days since the shooting, officials said.

“All threats against members of the NYPD are taken seriously and are investigated immediately to determine the credibility and origin of the information,” the department said in a statement.

NYPD Sgt. Ed Mullins, president of the Sergeants Benevolent Association (SBA), issued a statement today noting that the department "received notice there would be a violent act committed at Precincts 73rd, 79th and 81st in Bedford-Stuyvesant," the Brooklyn neighborhood where Officers Rafael Ramos and Wenjian Liu were slain Saturday.

"This potential threat is currently under review, and as a result, members of the Emergency Service Unit (ESU) team have been assigned to ensure the safety of the general public and our officers in and around the precincts. We take every threat seriously," Mullins said, adding, "we refuse to let threats trump the goodwill and spirit of the holidays."

“Any statement suggesting violence towards a police needs to be reported to the police so we can stop future tragedies,” de Blasio said Monday. “That is our obligation.”

In a letter to NYPD patrolmen, obtained by ABC News, union president Pat Lynch reminded officers safety is their first priority.

“The assassination of Police Officers Ramos and Liu has once again underscored a grim reality of our job: our uniform makes us a target,” he said.