The ambush-style slaying of five police officers during a protest in Dallas, marks the deadliest single attack on officers since Sept. 11, according to the National Law Enforcement Officers Memorial Fund. 

A sniper ambushed police officers Thursday, killing five and wounding seven officers and two civilians during a protest against police-involved shootings in downtown Dallas. 

According to Dallas police, three suspects are in custody and a fourth suspect who holed himself up in a community college garage early Friday was killed by police using a robot-controlled explosive device. 

President Obama, who is Poland for a NATO Summit, condemned the sniper-style shootings of Dallas police officers Friday, calling them a "'vicious, calculated,  despicable attack on law enforcement."

The deadliest day for law enforcement in the U.S. continues to be the 9/11 terror attacks, where 71 officers lost their lives, responding to the attacks on the World Trade Center, according to the Fund, which has data dating back to 1857 on law enforcement slayings. 

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Contributing: Gregory Korte, USA TODAY, and WFAA-TV, Dallas.