In 2013, some of the year's biggest headlines involved cop killers.

There was MIT Police Officer Sean Collier, who authorities said was shot and killed by the Tsarnaev brothers after the Boston Marathon bombings in April.

There was Christopher Dorner's vengeful rampage across Southern California in February. Two of the four people he killed were police officers.

Yet, those tragedies would prove the exceptions in 2013, which may have been one of the safest years in modern history for American police.

According to a preliminary report by the National Law Enforcement Officers Memorial Fund released Monday, 111 law enforcement officers died in the line of duty in 2013, the lowest number since 1959.

Most of the deaths came in traffic accidents, said the nonprofit, which tracks law enforcement deaths and maintains the National Law Enforcement Officers Memorial in Washington.

The most dramatic number in the report may be the 33 officers killed by gunfire, the lowest number since 1887, when the country's population was more than four times smaller than it is today.

It's also a steep drop from two years ago, when 71 officers were shot and killed on the job. A majority of 2013's shooting deaths came from handguns, most often during an ambush.

Such a fatal attack happened last week, when a suspected bank robber killed a police officer and critically wounded another officer on a roadway in Tupelo, Miss., authorities said. The suspect, Mario Edward Garnett, 40, was killed in a shootout with Phoenix police after another bank robbery in Arizona on Saturday.

The year's 46 police roadway deaths were the lowest number since 1991, the fund said. Eleven of those deaths came from officers being hit while outside their vehicles.

"The only good news is zero deaths, but this very significant drop in law enforcement fatalities the past two years is extremely encouraging," the fund's chairman and chief executive, Craig W. Floyd, said in a statement. "Our organization, in partnership with others, is working hard to create a new culture of safety in law enforcement that no longer accepts deaths and injuries as an unavoidable part of the job."

The 2013 number continued a general decades-long decline for law enforcement deaths since a peak of 280 deaths in the early 1970s.

But the number of justified homicides by police officers has increased, according to the latest available statistics from the FBI, which do not include 2013.

Officers killed 414 people in the line of duty in 2009; 397 in 2010; 404 in 2011; and 410 in 2012.