Police in major U.S. cities and suburbs are intensifying their intelligence-gathering on street gangs -- including some from Long Island and New York City -- whose members are considered most likely to launch copycat attacks after the murder of two NYPD officers, according to law enforcement sources.

Members of dozens of neighborhood "sets" affiliated with drug-dealing gangs, such as the Bloods, Crips, MS-13, the Black Guerrilla Family and several smaller crews, are being monitored through on-the-ground surveillance, undercover investigations, social media tracking and close cooperation with informants, the sources said.

Some of the gang members being tracked have been among the thousands of demonstrators who have taken to the streets to protest police violence and the killings of Michael Brown and Eric Garner, the sources said. Among them are gang members believed to have fired guns and damaged property during looting and clashes with police in Ferguson, Missouri, the sources said.

Other gang members being monitored have used "language that is meant to incite violence" against police in the five boroughs, Nassau and Suffolk counties and upstate, one police official said.

"Agencies are sharing intelligence and keeping each other apprised of threats that could cross county or city lines," said the source, who, like other law enforcement officials, spoke on condition of anonymity because he was not authorized to publicly discuss the issue. "The gangs are a priority, because hating cops is part of gang culture. In many cases, they also have the weaponry to go with that anti-law-enforcement attitude."

NYPD killings up urgency

Efforts to more closely monitor threats posed by gangs were bolstered after the execution-style shootings of NYPD Officers Wenjian Liu and Rafael Ramos in Bedford-Stuyvesant on Dec. 20. Ismaaiyl Brinsley, 28, shot the officers as they sat in their patrol car, then fatally shot himself in a nearby subway station, police said. NYPD Commissioner William Bratton described the killings as an "assassination."

In addition to focusing on gang members, many of whom earn money through selling drugs and committing robberies, thefts, or other crimes, investigators are keeping tabs on members of anarchist groups espousing violence against the police, as well as "extremists" within the Occupy Wall Street movement who have called for anti-Semitic violence, anti-police violence and property damage, the sources said.

Authorities acknowledge facing a complicated task as they try to head off potential violence by gang members and radical protesters. Their challenge: stay abreast of the activities of those whose criminal histories, rhetoric or public actions suggest they might carry out attacks on civilians, police or property, while also respecting civil liberties.

An overwhelming majority of protesters in New York and across the country have demonstrated peacefully after grand jury decisions not to indict police officers in Missouri and Staten Island.

Protest complications

But at least several dozen people who attended protests have "gone beyond the pale in terms of their activities or level of incitement," a second police official said. Those protesters -- some of whom carried signs or chanted slogans comparing police to the Ku Klux Klan -- have landed on the radars of law enforcement agencies in New York, Missouri, California, and elsewhere, according to a federal law enforcement official with knowledge of the intelligence-gathering efforts.

Arthur Eisenberg, legal director of the New York Civil Liberties Union, said the law requires that police have a "reasonable indication" of criminal activity before they start an investigation into political groups, which includes demonstrators.

"At the very least, that means they have to have some evidence of illegal activity that they think has taken place or is about to take place," Eisenberg said. "They cannot rely purely on rhetoric or advocacy. They don't need a level of evidence that will satisfy a probable cause standard, but they do need evidence."

Dozens of cases

The NYPD as of Dec. 25 had received 47 threat cases from a variety of law enforcement agencies and online sources, as well as 911 and 311 calls, officials said. About 24 of the cases were closed, 15 remain open and eight were referred to other law enforcement agencies.

The threats have led the department to station officers outside precincts to guard against potential attacks. As of Dec. 26, the NYPD had charged at least six people with either making terroristic threats against police or calling in false reports of threats.

In a statement issued Dec. 24, the NYPD said its "officers are advised to remain vigilant at all times. Additionally, security measures will continue to be assessed and police resources will be deployed accordingly. However, we continue to ask that the public, upon becoming aware of any threats, immediately report the information to the police."

The federal law enforcement official said investigators in New York and across the country are seeing a "normalization" of the type of anti-police rhetoric that, in the past, was more traditionally associated with street gangs, anti-government extremists and other fringe groups.

"What we're seeing is that anti-police sentiment that's enshrined in gang life becoming more mainstream," he said. "It's not a positive trend."