An ambush is generally referred to as an attack that takes place by surprise or from a concealed position. The National Law Enforcement Officers Memorial Fund is a not-for-profit organization based in Washington, D.C. It was established in 1984 to raise awareness about policing issues and honor officers’ service.

The fund says that on average one law enforcement officer is killed in the line of duty in the U.S. every 61 hours and that more than 20,000 police officers have lost their lives in the line of duty since the “first known line-of-duty death in 1791.”

Overall, as of July 20, 67 law enforcement officers had died in the line of duty in 2016, according to the fund. That figure represents a small increase over the 62 deaths for the same period in 2015. Thirty-two of those 67 deaths were firearms-related, according to the report.