Trump shooting updates: Shooter flew drone 200 yards from site hours before shooting

The Department of Homeland Security and the FBI have issued a bulletin to law enforcement across the country warning them that, in the wake of Saturday's shooting, violent extremists could try to conduct "follow-on or retaliatory" attacks at events over the next few months related to the 2024 presidential election.

The bulletin urged local, state and federal law enforcement -- and their partners -- to remain vigilant and report any suspicious behavior.

The bulletin also said that "lone actors" or small groups of extremists "likely will continue to view" political rallies and campaign events as "attractive targets."

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Thomas Matthew Crooks, the 20-year-old suspected gunman, planned to attend Robert Morris University this fall, according to a school spokesperson.

Robert Morris University is a private, four-year university located outside of Pittsburgh.

"We have been in touch with law enforcement and stand ready to assist in their investigation," school spokesperson Brian Edwards said.

Crooks had earned his associate degree in science from a local community college and graduated in May with high honors.

He was accepted into both RMU and the University of Pittsburgh, a public-research university also a close drive from his home in Bethel Park, Pennsylvania.

A spokesperson for the University of Pittsburgh said Crooks was told in February that he could attend this fall semester as a transfer student. In March, Crooks informed Pitt that he would not attend.

-ABC News’ Anne Flaherty

The suspected gunman who attempted to assassinate former President Donald Trump at his campaign rally Saturday purchased 50 rounds of ammunition ahead of the event, according to a bulletin from the Department of Homeland Security and the FBI Monday.

Thomas Matthew Crooks purchased the rounds from a gun and ammunition shop in Bethel Park, Pennsylvania, according to officials.

The bulletin notes that two improvised explosive devices were found in the shooter's car and one at his home.

"Over the last several months, Crooks received several packages, including some marked as possibly containing hazardous material, according to a review of his shipping history," officials wrote.

Officials also warn of future "follow-on or retaliatory acts of violence in response to this assassination attempt."

"We cannot rule out the possibility that some DVEs (Domestic violent extremists) or other actors may attempt follow-on or retaliatory acts of violence in response to this assassination attempt."

-ABC News' Mike Levine