The F.B.I. released alarming data showing a rapidly escalating pattern of public shootings in the United States on Monday, one day before the massacre in Uvalde, Texas.

The bureau identified 61 “active shooter” attacks in 2021 that killed 103 people and injured 130 others. That was the highest annual total since 2017 when 143 people were killed, and hundreds more were wounded, numbers inflated by the sniper attack on the Las Vegas Strip in October of that year.

The 2021 total represented a 52 percent increase from the tally of such shootings in 2020, and a 97 percent increase from 2017, according to the F.B.I.’s Active Shooter Incidents in the United States in 2021 report.

The report classifies an “active” shooting incident as one in which “one or more individuals actively engaged in killing or attempting to kill people in a populated area.”

The Gun Violence Archive, a nonprofit organization that tracks shootings around the country, defines a “mass shooting” as an incident in which four or more people are shot or killed, without differentiating between people who attack strangers or people they know, or the setting of the crime.

The archive has counted at least 215 such shootings through mid-May. Of those shootings, 10 involved four or more fatalities. The group recorded 693 mass shootings last year, with 28 involving four or more fatalities.

The deadliest mass shooting last year occurred at a grocery store in Boulder, Colo., where 10 people were killed, according to the F.B.I.

The shooting at Robb Elementary School in Uvalde, Texas, has already surpassed that, with at least 18 reported dead; Earlier this month, a white supremacist stalked and killed 10 people in a supermarket in Buffalo.

The F.B.I. report identified one particularly chilling trend among active shooters: Officials have noticed an increase in the number who moved from place to place in search of victims, a group the bureau refers to as “roving” shooters.

More than two dozen shootings in 2021, including eight murders at an Atlanta spa, fit that category.

Among the report’s other main findings:

• All but one of the 61 active shootings last year were carried out by men, ranging in age from 12 to 67; two wore body armor; 30 shooters were apprehended by law enforcement, 14 shooters were killed by law enforcement, four were killed by armed citizens, 11 shooters committed suicide, and one remains at large.

• The report is part of a series of FBI active shooter-related “products” first published in 2014, intended to provide insight for first responders, policymakers and law enforcement officers.

• The guns used in mass shootings are usually, for the most part, purchased legally — although many are later illegally modified to increase their firing rates and bullet capacities.

• From 1966 to 2019, 77 percent of mass shooters obtained the weapons they used in their crimes through legal purchases, according to a comprehensive survey of law enforcement data, academic papers and news accounts compiled by the National Institute of Justice, the research wing of the Justice Department, and released earlier this year.