Two police officers shot and wounded while standing guard outside the Ferguson, Missouri, police department early Thursday were deliberately targeted in what a police official called an "ambush."

Such ambush-style attacks were the leading method in the surging number of shooting deaths of law enforcement officers, according to the nonprofit, Washington-based National Law Enforcement Officers Memorial Fund (NLEOMF).

The shootings were a chilling low point in the nonstop protests in the city since a Ferguson police officer shot and killed 18-year-old Michael Brown in August.

The number of law enforcement officers shot to death in the line of duty rose more than 50% in 2014, NLEOMF said in a December report.

Fifty officers were killed by firearms -- 15 in ambush attacks -- in 2014, compared with 10 ambushes among 32 shooting deaths the year before.

"With the increasing number of ambush-style attacks against our officers, I am deeply concerned that a growing anti-government sentiment in America is influencing weak-minded individuals to launch violent assaults against the men and women working to enforce our laws and keep our nation safe," Craig W. Floyd, chairman and CEO of NLEOMF, told CNN.

"Enough is enough," Floyd said. "We need to tone down the rhetoric and rally in support of law enforcement and against lawlessness."