Tuesday morning's ambush-style killing of two police officers in Iowa is just the latest example in a growing trend of citizens fatally targeting members of law enforcement. Scott Michael Greene was arrested later Tuesday as the shootings' prime suspect, and his alleged motivation was right in line with some of the others charged with similar crimes: lone wolf gunmen with animosity for police.

The shootings separated by just minutes in Des Moines targeted officers sitting in their patrol cars. The killings may have been spurred by a recent confrontation Greene had with police as well as other previous contentious interactions with law enforcement, but an official motivation was not immediately announced.

Fatally ambushing police officers has seen a 300 percent increase this year compared to the first half of last year, USA Today reported. While those ambush-style deaths occurred under a number of high-risk circumstances, including "executing a tactical arrest," many of the more recent high profile instances were not.

Below is a list of some of the more notable instances of police being killed after they were ambushed.

Palm Springs, October 2016

Three unsuspecting police officers in Palm Springs, California were shot, two fatally, when a man fired a gun through a home door because he "wanted to kill police officers," Riverside County District Attorney Mike Hestrin said days after the Oct. 8 shootings. "That's the motive. He wanted to gun down police officers because they wore the uniform."

Philadelphia, September 2016

A man shot an officer sitting in her patrol car before shooting and killing another officer in Philadelphia Sept. 17. The gunman, who was eventually killed by police, had written a "rambling" note ahead of shootings indicating his disdain for law enforcement, CBS News reported.

Dallas July 2016

Five Dallas police officers were killed July 7 when a sniper took aim at them in an apparent act of racial revenge over the spate of law enforcement shooting black people. The lone gunman "wanted to kill [police] officers," Dallas Police Chief David Brown said at the time.

Baton Rouge July 2016

Less than two weeks after the Dallas shootings, a man fatally ambushed three officers in Baton Rouge, Louisiana July 17 because of a police-involved shooting of an unarmed black man in the city earlier that month. While a motive for the killings was never officially established, "There is no doubt these officers were intentionally targeted and assassinated," Louisiana State Police Col. Mike Edmonsdson said at the time. "It was a calculated act against people who work to protect this community every day."

Aside from this year, there were two other high profile instances of police being ambushed as they sat defenseless in patrol cars in recent years.

Brooklyn, December 2014

Two New York Police Department officers were "assassinated" by a Baltimore man who drove to New York City Dec. 21, 2014 to exact revenge for the police killings of Eric Garner, in Staten Island, and Michael Brown, in Ferguson, earlier that year. "I’m putting wings on pigs today," gunman Ismaaiyl Brinsley posted to his Instagram account hours before the shootings. "They take 1 of ours ... let’s take 2 of theirs."

Boston, April 2013

The brothers accused of carrying out the deadly Boston Marathon bombing in 2013 ambushed a police officer at a local college while they were on the run from authorities April 18, 2013. Dzhokhar Tsarnaev and his brother Tamerlan Tsarnaev were accused of shooting Massachusetts Institute of Technology police officer Sean Collier in the head while he was sitting in his patrol car just days after they detonated bombs along the race's finish line, killing three people.