WASHINGTON — In unscheduled and unusually personal remarks, President Barack Obama tried Friday to explain why African-Americans were upset about last week’s acquittal of George Zimmerman in the shooting death of Trayvon Martin while lowering expectations for federal charges in the case.

“Trayvon Martin could have been me 35 years ago,” Pres. Obama told White House reporters in a surprise appearance at the daily briefing. His remarks, which lasted about 20 minutes, escalated a nationwide debate on the verdict that has prompted protests, including some that turned violent.

A Florida jury acquitted Zimmerman last Saturday in Martin’s February 26, 2012, shooting death, inciting anger among many who considered the incident racially motivated murder. Pres. Obama issued a written statement on Sunday, noting that the jury had spoken and urging calm and reflection. Despite some calls for him to speak about the case, the nation’s first African-American president had made no further public comment until Friday.