Ambush attacks against law enforcement officers remain a threat to officer safety, with the number of attacks per year holding steady at about 200 a year since a decline in the early 1990s, according to a Department of Justice study released Tuesday.

The study by the department’s Office of Community Policing, examined the ambushes – or planned surprise attacks —  of law enforcement officers between 1990 to 2013 and concluded that concerns about targeted violence against police were on the rise and that “officers must not only be guardians of the public but also be prepared to respond to violence targeting them.”

Although the number of ambush attacks on police per years is steady, the report says that the proportion of fatal attacks on officers attributable to ambushes is increasing — during the years they examined.

“Law enforcement officers regularly put their lives on the line in order to protect our communities and serve our nation,” said Attorney General Loretta Lynch, in releasing the report, titled “Ambushes of Police: Environment, Incident Dynamics, and the Aftermath of Surprise Attacks against Law Enforcement.”

“As part of our work to support these brave men and women, the Department of Justice is committed to extensive efforts aimed at preventing violent action against the police,” Lynch said in a statement. “This report will serve as a critical base of knowledge as we work to defend our law enforcement and ensure our officers’ safety.” Lynch was scheduled to travel to Chicago Tuesday and address the International Association of Chiefs of Police, but had to cancel her trip because she was ill.

Some of the conclusions are not surprising. The report found that areas where crime is high and assaults against police are high “may be more prone to severe attacks such as ambushes.” The study found that progressive hiring practices and standards were associated with a lower number of ambushes. Technology, especially in-car cameras, significantly lowers ambush assaults and is a potential deterrent on both officer and citizen, the report found.

But, oddly, the report also found a small and “confounding” correlation between police ambushes and community oriented policing.

“We found that community policing…had the opposite effect than what we had expected,” the report said. “It is not within the realm of any plausible theory that community-oriented policing is a contributing factor to violence against the police.” But it was unclear whether the correlation was because of incomplete data or because communities that have significant violence already have responded with community policing activities, the report said.

Ronald Davis, director of the Justice Department’s Office of Community Oriented Policing Services said the ambush report “is an important first step.”

“We know that the murder of a police officer in the line of duty is an assault on the entire community,” Davis said. “When that murder is a result of an ambush, it also attacks the very foundation of our democracy.  We must act to address this persistent threat.”

But the report recommended that data collection methods be improved and standardized to provide a better understanding of the national trends in ambushes of law enforcement officers, and concluded that more research is needed to assess the impact of law enforcement practices and operations on violence against the police.