The mass shooting at a Dallas protest that left five officers dead and six others wounded is the deadliest attack on law enforcement since 9/11, according to statistics from the National Law Enforcement Officers Memorial Fund. Dallas Police have confirmed that four of their police officers have died, and Dallas Area Rapid Transit (DART) has confirmed that one DART officer also died. DART has identified him as Officer Brent Thompson, 43. He was the first DART officer to be killed in the line of duty since the agency's formation. The deadliest day for U.S. law enforcement in history was 9/11, in which 71 officers died while responding to the terror attacks on the World Trade Center in Lower Manhattan -- including 37 members of the Port Authority of New York and New Jersey Police Department, which represented the single largest loss of law enforcement personnel by a single agency in U.S. history, according to the Fund. SUBSCRIBE to ABC NEWS: https://www.youtube.com/ABCNews/ Watch More on http://abcnews.go.com/ LIKE ABC News on FACEBOOK https://www.facebook.com/abcnews FOLLOW ABC News on TWITTER: https://twitter.com/abc GOOD MORNING AMERICA'S HOMEPAGE: https://gma.yahoo.com/