Mass killings in the U.S., like Monday's assault at the Navy Yard in Washington, have become a troubling and recurring fact of life in America.
In the latest carnage, authorities say a gunman killed 12 people, many of them office workers, before being killed in a shootout with police. National television broadcasts captured the frantic scenes a short distance from the U.S. Capitol, with workers running from an office building in fright and police armed with assault rifles looking for the killer.
It was not the only mass killing this year in the U.S., but it was the worst. The country's Federal Bureau of Investigation defines a mass killing as an incident where four or more people are killed, not including the killer. By that measure, there have been 19 other such incidents in the U.S. in 2013 - often times family disputes that draw little national attention - and a total of more than 200 such cases since 2006.
Last December, a gunman killed his mother, then drove to a nearby elementary school in the northeastern state of Connecticut and killed 26 people, including 18 students.
Before that, in July 2012, a troubled graduate student opened fire at a midnight showing of the latest Batman movie at a Colorado theater, killing 12 people. Less than a month later, an Army veteran killed five men and a woman at a Sikh temple in Wisconsin.
Like Monday's assault, the killings have often occurred in seemingly peaceful settings. A gunman in early 2011 killed six people and wounded 13 others, including a U.S. congresswoman, as she was meeting with voters on a Saturday morning outside a grocery store in Arizona.
In 2009, an Army psychiatrist killed 13 soldiers and civilians on an Army base in Texas. He was recently sentenced to death, but mandatory reviews of his sentence could take years.
Two years earlier, a student at a large university, Virginia Tech, killed 32 people on the sprawling campus. In 1999, two students at a Colorado high school killed 12 of their classmates and a teacher.
After mass killings in the U.S., some lawmakers have called for much tighter gun controls. Last year's Connecticut schoolhouse slaughter sparked an extensive debate in the U.S. over gun rights, and the U.S. Constitution guarantee of the right to bear arms.
U.S. President Barack Obama called for stiffer gun controls, but Congress rejected new restrictions.