A 64-year-old District man pleaded guilty Friday to robbing 11 banks in the city in a 16-month spree that ended with his arrest in March.

Jerome Thomas, who typically claimed to have a gun or a pipe bomb but never showed a weapon, also admitted trying to rob a twelfth bank. Each of the dozen counts against him carries a possible sentence of 20 years in prison, authorities said.

Beginning Nov. 23, 2009, Thomas robbed a half-dozen Chevy Chase Bank branches, three branches of PNC Bank and one branch each of Capital One and SunTrust banks, the U.S. attorney’s office said. He also attempted to rob a fourth PNC branch.

“Mr. Thomas repeatedly placed innocent victims in fear for their lives and will pay the price with time behind bars,” said James W. McJunkin, a top official in the FBI’s Washington field office. He is to be sentenced Aug. 25 in U.S. District Court in Washington.

“This case proves beyond a shadow of a doubt that crime does not pay,” U.S. Attorney Ronald C. Machen Jr. said. “In nearly a dozen bank robberies, this prolific bank robber netted just $22,000.”

Sentencing is scheduled for August 25.