The number of ambushes of police officers has been creeping back up in recent years, creating concern among law enforcement officials at a time when relations between police and the communities they serve are particularly fraught.

The number of surprise assaults on police rose to about 250 incidents per year between 2008 and 2013, up from about 200 per year during the previous 10-year period, according to a Justice Department study released Tuesday. Those figures were still far lower than in the 1990s, when ambushes peaked one year above 500.

The recent uptick has concerned officials. “In an era of strained community relations and struggles with police legitimacy, violence against police is of particular concern,” Ronald L. Davis, director of community-oriented policing at the Justice Department, told the Los Angeles Times.