New Zealand's prime minister said this is one of her country's "darkest days," after 49 people were killed in an attack on Muslims at prayer. Worshipers at two mosques were gunned down Friday in Christchurch, the country's second largest city.

The main suspect is a white Australian man who has been identified as Brenton Tarrant. He has been charged with murder. Three others have also been taken into custody in connection with the deadliest mass shooting in New Zealand's history.

Video that was apparently livestreamed on social media by the shooter shows the attack in horrifying detail. New York's deputy head of counterterrorism John Miller told "CBS This Morning" this attack bears many similarities to ISIS.

"You can see that the neo-fascist groups, white supremacists are borrowing from the ISIS playbook," Miller said. "In this attack you see the phenomenon that we coined the phrase in the NYPD 'dying live.' This was an ISIS tactic where they said if you're going to do a mass casualty attack, you should live stream it over social media. We've seen them adopting many of the tactics, the terrorist tactics you would see in things like ISIS' Rumiyah magazine or Al Qaeda's magazines in terms of instruction."

A 74-page racist manifesto was posted online by a man who said he was behind the Christchurch attacks. CBS News has not been able to confirm if it was posted by Tarrant. But it lays out the alleged motive, citing anti-immigrant sentiment and revenge for past terror attacks in Europe.

Asked where the threat from white nationalists falls in the NYPD's threat matrix, Miller said, "You saw the attack on the mosques in Canada, an active shooter attack. You see the Pittsburgh synagogue attack. You see some of the propaganda and online forums there. So this is something that we monitor very carefully. It's something that has been emergent. We're seeing an increase in the propaganda. Again, when we look at their propaganda, they are borrowing propaganda techniques from other terrorist groups. So this is something we watch closely."

Miller said that because the NYPD has a post in Australia the information about the attack was coming to his department in real time overnight.

"Very quickly we identified we're going to have to increase police experience around mosques, around houses of worship. We're going to have to have roving patrols," he said. "Our house of worship [patrol] cars that are assigned to the precinct to keep the liaison with religious institutions are going out and making contact. You'll see a public message of reassurance."

Law enforcement in New Zealand said the suspect is claiming there are others planning attacks on mosques in the works. Miller said that may or may not be accurate but it is something they will have to take seriously. As for whether there's any U.S. connection, Miller said, "what we see is no significant U.S. nexus."

"More importantly from our individual perspective, no nexus to New York City," he said. "We're going to provide police resources, reassurance, protection, high-profile resources, and we tell everybody, you don't cancel anything otherwise they win."