Multiple law enforcement agencies, including the FBI, were spotted in Weslaco as the agency investigates a deadly shooting at a border Patrol building in McAllen that left the gunman dead.

Three other people were injured in the shooting, authorities said.

The law enforcement activity was seen near South Bridge Avenue in front of the Dollar General at 518 S. Bridge Ave.

Channel 5 News reached out to the FBI, and a spokesperson confirmed that federal agents were in the area to conduct "court authorized law enforcement activity" in connection with the ongoing investigation of the shooting incident at the Rio Grande Valley Border Patrol Annex in McAllen.

"We are in the initial stages of the investigation and will pursue all logical leads while employing every investigative tool at our disposal to determine the motives behind Mosqueda’s actions," an FBI spokesperson told Channel 5 News in an email.

The shooting was reported on Monday morning at around 5:53 a.m. when a caller reported "30-60 shots" were heard at the annex building, and that a man wearing a mask was walking toward the building at 2301 S. Main St.

READ MORE: Gunman dead after opening fire at McAllen Border Patrol facility

The shooter, identified as 27-year-old Ryan Louis Mosqueda of Michigan, died after federal agents returned fire.

According to the McAllen Police Department, Mosqueda was reported missing from a Weslaco address around 4 a.m. Monday. 

Weslaco police spokesman Heriberto Caraveo confirmed to Channel 5 News that Mosqueda's father reported him missing.

According to Caraveo, Jose Mosqueda told Weslaco police that his son had been missing since Monday at 2:30 a.m. and had "mental deficiencies." Caraveo said there were no records to confirm that, and Mosqueda was not on any medication.

The investigation continues.