A federal grand jury has indicted a D.C. man on charges that he made two separate bomb threats against Amtrak, U.S. Attorney Ronald C. Machen Jr.’s office announced Friday.

The grand jury indicted Michael J. Dennis, 27, on two counts of threatening to blow up railroad carrier equipment and two counts of using a cellphone to make a bomb threat. If convicted, Dennis could face a sentence of up to life in prison.

The charges stem from threats Dennis made Nov. 30, 2011, and Jan. 19, 2012, against the New York Avenue Bridge. That bridge carries New York Avenue over the Amtrak, CSX and Metro tracks just north of Union Station. The threats caused numerous delays and affected 1,000 Amtrak passengers, according to Machen’s office.

“Threats against our public transportation systems are deadly serious,” Machen said. “They cause delays, distract attention from real public safety concerns, and instill unnecessary fear in the traveling public. This prosecution demonstrates our commitment to punishing any person who makes a threat against public transportation.”