The first target of anti-government extremists of any ilk is police.

NYPD officers Rafael Ramos and Wenjian Liu, were ambushed and killed. The gunman, Ismaaiyl Brinsley doesn't at this time appear to be connected to any group.  Something that needs to be kept in mind is that the last NYPD killed this way was in 1988. 
SEE:

The New York Times 

Two N.Y.P.D. Officers Are Killed in Brooklyn Ambush; Suspect Commits Suicide 

By BENJAMIN MUELLER and AL BAKERDEC. 20, 2014

Police are often the first target of right wing extremists. Sometimes police are shot by extremists at traffic stops and sometimes police are deliberately targeted by extremists. 

“In the past five years alone, from 2009 through 2013, ADL has tracked 43 sep­a­rate vio­lent inci­dents between domes­tic extrem­ists (of all types) and law enforce­ment in the United States. These inci­dents include sit­u­a­tions in which shots are exchanged between police and extrem­ists (shootouts), sit­u­a­tions in which extrem­ists have fired at police but police sub­dued the extrem­ists with­out hav­ing to return fire, and sit­u­a­tions in which offi­cers had to use their firearms to pro­tect them­selves against extremists.

Of these 43 inci­dents, fully 39 of them involved extrem­ists sport­ing some sort of extreme right-wing ide­ol­ogy. White suprema­cists took part in 21 inci­dents, while anti-government extrem­ists were involved in 17 more. An anti-Muslim extrem­ist was involved in one inci­dent (the other four inci­dents included one with a left-wing extrem­ist and three with domes­tic Islamic extrem­ists). In these shoot­ing inci­dents, the extrem­ists shot 30 offi­cers, 14 fatally. Many other offi­cers sus­tained non-gunfire injuries dur­ing some of these encounters.” 

FROM: 

JUNE 9, 2014 

ADL Extremism & Terrorism

 Officers Down: Right-Wing Extremists Attacking Police At Growing Rate

SEE:

SPLC - Southern Poverty Law Center

Law Enforcement Resources

Home » What We Do » Hate and Extremism

"Law enforcement professionals are more likely to encounter dangerous extremists than virtually any other segment of American society — and those confrontations are, tragically, sometimes fatal. In fact, a 2010 University of Maryland study found that 49 of more than 400 people killed by radical-right extremists since 1990, or nearly 15%, have been law enforcement officers. With that in mind, the SPLC has undertaken a number of initiatives to equip officers with information and other resources that may help them carry out their duties with a minimum of danger to themselves.

Our free law enforcement trainings teach officers how to recognize hate groups, symbols and activity; the threat potential of specific groups; and how to respond to hate group activity. The Intelligence Files contains updated biographical profiles of leading hate groups and extremist leaders, plus background on the various extremist ideologies. And our Hate Map helps officials locate extremist groups within their communities."

http://www.splcenter.org/what-we-do/hate-and-extremism/law-enforcement