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Once he was freed from jail and the charges against him were dropped, Pedro Hernandez expected to get a fresh start.

He had spent a year in jail rather than plead guilty to a shooting in 2015 that he said he did not commit, and his case had become a rallying point for supporters of changing bail laws.

Eventually the Bronx district attorney dropped charges against him in the shooting and a separate schoolyard assault. By December, he was set to go to college, and even had a meeting with former President Barack Obama on his schedule.

But on Sunday night, Mr. Hernandez, 19, was arrested again in the Bronx. The police said he was involved in a robbery and knife attack that left one man with a gash on his face that required 15 stitches.