Authorities checked bags, combed through debris and flooded the streets with investigators Tuesday in a rush to determine who set off two shrapnel-studded bombs at the Boston Marathon that killed three people and injured at least 144.

Law enforcement officials went to a suburban apartment building, interviewed two men and removed a duffel bag and two trash bags overnight. They also issued alerts for a rental van and a man seen leaving the scene of the blasts.

But the significance of the police activity, if any, was not clear one day after the twin blasts, which detonated seconds apart and turned the finish line of the marathon into a hellish scene of panicked spectators, shattered glass and blood-spattered sidewalks.

Law enforcement officials told NBC News that the bombs were packed with ball bearings and BBs, apparently intended to increase the casualties.

Among the dead was an 8-year-old boy, identified by NBC affiliate WHDH as Martin Richard, who was waiting at the finish for his father to finish the race. Among the injured were brothers, 33 and 31, who each lost a leg from the knee down, The Boston Globe reported