Following the killing of the gunman who tried to assassinate former President Donald J. Trump on Saturday during a rally in Butler, Pa., investigators discovered two improvised explosive devices in the would-be assassin’s car that used a radio-controlled initiation system designed for commercial fireworks demonstrations.

Details on the improvised bombs were in a document released by a federal government task force on Monday to law enforcement departments across the country.

The New York Times obtained a copy of the document, called a “quick look” report, on Tuesday.

The two devices in the car owned by Thomas Matthew Crooks, 20, of Bethel Park, Pa., were almost identical in construction, according to the task force’s report. Each contained a cardboard tube filled with a gray powder that was described by a police bomb technician as having “an odor of nitromethane” and appeared to contain prills — essentially small pellets.

While the report does not contain a laboratory analysis of the gray powder, the description suggests the presence of a fertilizer, which is often packaged in prill form. According to the National Institute for Health, nitromethane is sometimes used as a fuel in racecars and has a “strong disagreeable odor.” It can be purchased online.

The combination of certain kinds of fertilizers and fuels is a common method for creating homemade explosives, such as the ammonium nitrate-based device Timothy J. McVeigh used to destroy the Alfred P. Murrah Building in Oklahoma City, Okla., in 1995, killing 168 people.

Whether or not Mr. Crooks attempted to detonate the devices is unclear, but the report described them in a manner that cast doubt on their viability as weapons.