A prominent member of the Department of Government Efficiency was beaten in an attempted carjacking in Washington this week, prompting President Trump to renew his threat of a federal takeover of the city.

The victim was Edward Coristine, a 19-year-old software engineer known by his online sobriquet, Big Balls, according to the police, who said he was surrounded and attacked by 10 young assailants outside his car.

In a social media post on Tuesday afternoon, Mr. Trump shared a photograph that appeared to show Mr. Coristine lying in the street bleeding, battered and shirtless, writing that crime in the nation’s capital was “totally out of control,” though the city’s crime rates have been falling.

“If D.C. doesn’t get its act together, and quickly, we will have no choice but to take Federal control of the City,” he said.

The president argued that young people in the city do not fear consequences if they commit crimes, tapping into a thorny local issue. Youth crime remains a trouble spot for D.C., with young people making up a majority of the arrests for robbery and carjacking. In April, Mayor Muriel E. Bowser announced the creation of a special police unit specifically dedicated to preventing and responding to juvenile crime.

“The Law in D.C. must be changed to prosecute these ‘minors’ as adults, and lock them up for a long time, starting at age 14,” Mr. Trump said in his social media post on Tuesday.