The 1920s was one of the deadliest decades in U.S. law enforcement history, with an average of almost 229 police officers killed annually, according to the National Law Enforcement Officers Memorial Fund. The violence paralleled Prohibition and the rise of bootleggers and gangsters, who often outgunned police. More police died in Chicago than anywhere else in the country. The crime and violence gave rise to greater cooperation between federal law enforcement, led by the FBI, and state and local police. The deadliest year was 1930, when 285 police officers were killed.

On Aug. 1, 1966, Charles Whitman, an ex-Marine trained as a sniper and a student at the University of Texas in Austin, killed 16 people and wounded dozens more in a rampage known as the Texas Tower shooting. The shootings, along with the tumult of the 1960s in general, spurred the creation of tactical units known today as Special Weapons and Tactics, or SWAT, teams, according to the National Tactical Officers Association. In 1967, the Los Angeles Police Department created what is widely considered the first major police SWAT team.

Along with the Prohibition period, the 1970s was another deadly decade in U.S. law enforcement history. In 1971, 15 New York Police Department officers and detectives were killed, including two officers by Black Liberation Army members. The killings led to a decades-long rise in the number of police officers nationwide. By the end of the decade, more than 100,000 officers had been added, climbing to 436,000, according to the National Law Enforcement Officers Memorial Fund.

Starting with the Miami cocaine trade in the late 1970s and expanding into the Los Angeles crack market, police battled notoriously violent drug gangs. While cops wielded revolvers, criminals tapped into an arms market that equipped them with assault rifles. Nationwide, gun-related crime climbed steadily through the late 1980s before peaking in the early part of the 1990s, according to the U.S. Justice Department. By the end of the decade, there are nearly 500,000 law enforcement officers in the country, according to the National Law Enforcement Officers Memorial Fund.

Shown on live television, a botched Bank of America robbery led to a firefight that CNN likened to a combat zone. The two suspects, dressed like commandos with body armor and an arsenal of automatic weapons, fired hundreds of rounds, initially outmatching the Los Angeles police, 10 of whom were injured during the siege.

The Columbine, Colo., shootings, carried out by students Eric Harris and Dylan Klebold, ended with 15 deaths, including their own. The massacre shocked the country, drew international media attention and led to changes in how police respond to so-called active shooters. The number of police in America reaches more than 725,000, according to the National Law Enforcement Officers Memorial Fund.

The terrorist attacks on New York and the Pentagon made Sept. 11, 2001, the deadliest single day in U.S. law enforcement history. The attacks – which killed nearly 3,000 people, including 60 officers from the New York and Port Authority police – signaled a shift from community-oriented officers walking the beat to an intelligence-led approach with an emphasis on all hazards, including terrorism.

Chechen separatists stormed a school in Beslan, North Ossetia, a republic of the Russia Federation, and held hostage more than 1,000 people, most of them children. More than 300 hostages — nearly 200 children — were killed during the three-day crisis after Russian security forces stormed the building.

In a coordinated attack that left 52 dead and more than 700 injured, terrorists detonated bombs during rush hour, targeting London’s mass transit system. London police responded by deploying covert sniper units that tracked suspected terrorists with orders to shoot to kill if it appeared a suspect had a bomb or refused to surrender.

In another coordinated attack of shootings and bombings, Islamists targeted multiple spots in the Indian city of Mumbai, including hotels, a railway station, a café and a Jewish center. The attacks left 164 dead, including nine attackers. The attack alarmed police officials in the U.S., who worried about similar coordinated armed attacks in cities here.

Maj. Nidal Malik Hasan, an Army psychiatrist, opened fire at the Fort Hood Army Base near Killeen, Texas, killing 13 and wounding 32. The worst attack on U.S. soil since 9/11, the rampage stopped after a local police officer shot Hasan in the chest