Apart from the 9/11 terrorist attacks on the World Trade Center, which killed 23 New York police officers, Saturday's shootings of three Pittsburgh officers ranks as one of the deadliest single-day incidents for law enforcement in 40 years.
According to the National Law Enforcement Officers Memorial Fund, 133 officers were killed in the line of duty in 2008, a 27 percent decrease from 2007 and the lowest number since 1960.
Other incidents that produced multiple police fatalities:
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• March 21, 2009: Four Oakland, Calif., police officers were fatally shot during two confrontations with the same man, once during a traffic stop and again when a SWAT team entered the apartment where the suspect was hiding. One officer survived for two days.
• Sept. 8, 2007: Three corporals from Odessa Police Department in Texas were shot while responding to a domestic violence call. Two died at the scene, the third four days later. The 58-year-old suspect was taken alive into custody.
• March 3, 2005: Four junior members of Canadian Royal Mounted Police were killed by a man while raiding a marijuana farm in rural Alberta. The 46-year-old suspect killed himself before he could be arrested.
• Nov. 22, 1994: An ex-convict opened fire at the Metropolitan Police Department headquarters in Washington, D.C., killing FBI Special Agent Martha Dixon of Mt. Lebanon, a second FBI agent and a police sergeant. The 25-year-old suspect was killed.
• Feb. 28, 1993: Four federal Bureau of Alcohol, Tobacco, Firearms and Explosives agents died during a raid at Branch Davidian cult in Waco, Texas. Six cult members died that day, and 85 others during the 51-day siege that followed.
• April 6, 1970: Four California Highway Patrol officers were killed in a restaurant parking lot shoot-out that started with a traffic stop in Southern California. One suspect killed himself before capture. The other was convicted of the murders; his death sentence was reduced to life in prison after the California Supreme Court overturned the state's death penalty.