WASHINGTON — A report released by the F.B.I. on Wednesday confirmed what many Americans had feared but law enforcement officials had never documented: Mass shootings have risen drastically in the past half-dozen years.

There were, on average, 16.4 such shootings a year from 2007 to 2013, compared with an average of 6.4 shootings annually from 2000 to 2006. In the past 13 years, 486 people have been killed in such shootings, with 366 of the deaths in the past seven years. In all, the study looked at 160 shootings since 2000. (Shootings tied to domestic violence and gangs were not included.)

Many of the sprees ended before the police arrived, the report said. In 44 of the 64 cases in which the F.B.I. was able to determine the length of the shooting, the gunfire lasted less than five minutes. Twenty-three shootings ended in less than two minutes. In 64 of the 160 total cases, the gunmen committed suicide.