NYPD officers having a moment of silence near the spot in Brooklyn where two of their colleagues were killed on Dec. 20, 2014. AP The New York City Police Department has arrested six people in the past five days for making threats against officers.

These arrests have come out of "about 40 threat investigations" the NYPD launched following the killing of two officers on Dec. 20 by Ismaaiyl Brinsley, who made social media postings tying the shooting to recent controversies involving the deaths of unarmed African American men at the hands of police officers in New York and Ferguson, Missouri.

"All threats against members of the NYPD are taken seriously and are investigated immediately to determine the credibility and origin of the information. So far, we have assessed hundreds of online postings and calls to 911 and 311 that have resulted in about 40 threat investigations, of which about half have been closed or referred to other agencies," the NYPD's public information office said in a statement sent to Business Insider on Thursday.

In light of the threats, the statement said "officers are advised to remain vigilant at all times."

"Additionally, security measures will continue to be assessed and police resources will be deployed accordingly," the statement said. "However, we continue to ask that the public, upon becoming aware of any threats, immediately report the information to the police."

The NYPD also provided information about the six people arrested for allegedly threatening officers in recent days.

According to the department, an 18-year-old man named Devon Coley was arrested Monday morning in Brooklyn and charged with two counts of making a terroristic threat. Officer Mason, who works with the NYPD's deputy commissioner for public information, said Coley posted "remarks and photos on Facebook regarding shooting members of the 73rd Precinct" in Brooklyn.

On Monday evening, the NYPD said a 52-year-old Manhattan man named Robert Bowman was arrested and charged with filing a false report. Mason said Bowman "made a bogus 911 call stating that people were going to come to Staten Island and shoot officers."

The NYPD also said a 16-year-old boy was arrested on Tuesday evening in Brooklyn and charged with making a terroristic threat. According to Mason, the teenager made "a Facebook post where he stated, 'Let's kill cops.'"

An hour after the teenager was arrested on Tuesday, the department said a 46-year-old Staten Island man named Roberto Labita was arrested and charged with filing a false report. Mason said he "made a false 911 call stating that someone made threats about killing cops."

On Wednesday, the department said a 41-year-old Manhattan man named Tyrone Melville was arrested and charged with making a terroristic threat and two difference counts of aggravated harassment. Reuters reported the police said Melville allegedly called the 84th precinct in Brooklyn where the two slain officers were based and asked if bullets were removed from the dead men so he "could kill more cops."

A little over three hours after Melville was arrested on Wednesday, the NYPD said a 26-year-old Brooklyn man named Jose Maldonado was arrested and charged with making a terroristic threat. According to Reuters, the department said Maldonado posted pictures of weapons and threats to kill police officers on his Facebook page.

Tensions have been high for police around the country since widespread protests began after an officer in Ferguson shot and killed an unarmed black teenager named Michael Brown in August. The demonstrations intensified last month after a grand jury declined to indict the officer.

The nationwide protests about police violence have also been fueled by an incident that occurred in New York in July where a man named Eric Garner died while being taken into custody for allegedly selling untaxed cigarettes on Staten Island. Garner's arrest was videotaped and the footage showed an officer seemingly placing him in a banned chokehold. Earlier this month, a grand jury declined to indict that officer, which led to a new round of protests in New York and around the country.

Brinsley referenced both Brown and Garner's deaths in social media posts prior to killing the two officers.

New York Mayor Bill de Blasio (D) discussed the deaths of the officers, Rafael Ramos and Wenjian Liu, in a speech on Monday. In those remarks, the mayor encouraged people to report any threats against the police.

"If anyone of bad intent threatens a police officer – on the internet or anywhere else – anyone who hears such a threat must deliver that information immediately to the NYPD, must turn that individual in," de Blasio said.

Ramos' funeral is set to be held on Saturday and Vice President Joe Biden is scheduled to attend. Plans for Liu's funeral have not yet been set.