There were 61 "active shooter" incidents in the United States last year, an increase of more than 50% from 2020 and more than twice as many as five years ago, the FBI reported.
The shootings in 2021 left 103 people dead and 140 wounded, excluding the gunmen, the FBI said in a report released on Monday.
The FBI defines an active shooter as "one or more individuals actively engaged in killing or attempting to kill people in a populated area."
An active shooter attack is not to be confused with a mass shooting, which is typically defined as a shooting that involves four or more victims and can take place in public or private spaces.
The report comes nine days after an 18-year-old gunman killed 10 people and wounded three others at a supermarket in a predominantly Black neighborhood of Buffalo, New York.
The FBI is investigating the shooting as a hate crime and an act of racially motivated violent extremism.
The FBI said its active shooter data for the past five years show an "upward trend," with the number of incidents rising from 40 in 2020 and 30 in 2017.
The number of casualties recorded last year, 243, was the third highest over the five-year period, with fatalities at their highest level since 2017.
The increase in school attacks and other mass shootings in recent years has fueled a national debate over solutions, from limiting access to guns to early intervention to stop would-be shooters.
Of the 61 shooters involved in last year's incidents, 11 committed suicide, 18 were killed by law enforcement or private citizens and the rest were arrested.
The two active shooter incidents with the highest number of casualties last year took place at a FedEx operations center in Indianapolis, Indiana, and at a Kroger supermarket in Collierville, Tennessee.
In the first incident, on April 15, 2021, the gunman killed eight people and wounded seven others before committing suicide.
In the Kroger supermarket shooting, on Sept. 23, 2021, the gunman shot and killed one person and injured 14 others before killing himself.
Among other high-casualty incidents were shootings at a high school in Oxford, Michigan, and a grocery store in Boulder, Colorado.