The number of people killed or wounded in mass shootings in the U.S. has increased dramatically in recent years, an FBI study has found., according to BBC News.

More than 480 were killed and 557 wounded in 160 "active shooter" incidents from 2000-13, it found.

Two-thirds of incidents ended before police arrived, though only one ended when an armed citizen not working as a security guard responded with gunfire.

The study was prompted by the 2012 massacre at a Connecticut school.

President Barack Obama, a proponent of stricter gun control, signed a law in 2013 requiring the U.S. justice department to investigate mass shootings.

In the wake of the 2012 killing of 20 children and six workers at Sandy Hook Elementary School, his other gun control proposals have largely failed, blocked by senators and congressmen allied with the powerful gun lobby.

The study defines "active shooter" incidents as scenarios in which an individual or individuals kill or attempt to kill people in a populated area with firearms.

All but two of the incidents studied by the FBI were committed by a single individual, and in only six instances were the "active shooters" female.

A large majority of these scenarios, 70%, occurred in businesses or educational environments.

In the first seven years studied, an average of six "active shooter" incidents occurred annually. The average rose to 16 per year in the last seven years.

The average number of deaths in such incidents was also higher over the last seven years, including 90 killed in 2012.

Other findings include:

-Almost 70% of incidents in which investigators could ascertain the duration of the attack were over within five minutes

-Fifty-six percent of the shootings ended at the gunman's initiative, whether by suicide, ceasefire or flight

-In 21 incidents (13.1%), unarmed civilians successfully stopped the gunman, including 11 by school employees or students

-Five incidents (3.8%) ended when armed individuals who were not police exchanged gunfire with the gunman - four security guards and a single armed bar patron