UNITED STATES—Police officers throughout the United States have been attacked in a high number of ambush style shootings in 2016, increasing the total amount of gun-related officer deaths from 2015.

The International Association of Chiefs of Police state four factors that constitute an ambush attack include: an element of surprise, a lack of provocation, a suddenness to the attack, and attackers who conceal themselves, their weapon or their intent. The National Law Enforcement Officers Memorial Fund, a national non-profit that tracks the death of police officers, shows the number of fatal ambush attacks this year are the highest in any of the past 10 years.

In Iowa, in the month of November, Urbandale Police Officer Justin Martin was shot and killed in an ambush attack and 20 minutes later a few blocks away, Des Moines Police Officer Anthony Beminio was also shot and killed. He was responding to the report of Officer Martin being shot. The suspect, Scott Michael Greene, was taken into custody. Both officers were killed while sitting in their patrol cars.

Last month, there were four shootings of police officers which occurred on the same day at different locations throughout the United States.

On November 20, the first of the four shootings occurred at 11:45 a.m., where San Antonio Police Detective Benjamin Marconi was shot twice in the head and killed during a traffic stop by 31-year old Otis Tyrone McKane. According to the Associated Press, McKane said he was angry about a custody battle and took it out “on somebody who didn’t deserve it.”

Later in the evening, a St. Louis police sergeant was shot in the face twice and hospitalized in critical condition. The officer was sitting in his car at an intersection when a vehicle pulled up and opened fire. The officer who was shot has not been identified, but Police Chief Sam Dotson said he was expected to survive in during a news conference. The suspect was killed following a late-night manhunt and shooting with police.

“It is certainly a coincidence, but we’re not going to venture to say they’re connected,” said Chief William McManus from the San Antonio Police Department.

The ambush shootings continued hours later in Sanibel, Florida, as Officer Jarred Ciccone was shot in the shoulder during a drive-by at a traffic stop. The suspect was taken into custody after exchanging gunfire with officers from the Sanibel Police Department. Ciccone was taken to the Lee Memorial Hospital where he was treated and released injuries he sustained.

On November 20, at about 10:30 p.m. in Gladstone, Missouri, an officer was shot in the hand. The suspect, Jacob Stevens, 18, was the passenger in a car that was pulled over. Police stated Stevens jumped out the car and tried to run away, and during the struggle Stevens pulled out a gun and shot the officer in the hand. Police fired back, shooting and killing Stevens. The officer had surgery and is expected to recover.

According to the National Law Enforcement Officers Memorial Fund, a total of 137 officers have died in the line of duty in 2016, ranging from vehicular assault, duty-related illness and other variety of accidents. Of the 137 officers that have fallen, 62 of them were killed in the line of duty from gunfire, an increase from last year’s firearm-related fatalities of 38.

The most recent incident involving police shootings involved two officers who were killed near Georgia Southwestern State University on December 8. One officer, Nicholas Ryan Smarr, was killed on the scene and the second officer, GSW Public Safety Officer Jody Smith, died from injuries he sustained.

“There is a clear and present danger for police officers right now and we’ve doubled up our safety precautions,” said Sgt. Paul Parizek, spokesperson for the Des Moines Police Department.