WASHINGTON President Joe Biden on Monday nominated former federal prosecutor Steve Dettelbach to head the agency overseeing guns and announced that new rules have taken effect to track so-called ghost guns kits that until now were not required to bear serial numbers.

All of a sudden its no longer a ghost, Biden said. It has a return address.

Biden, who has been criticized by gun-control advocates for not doing more to address gun violence in his year in office, invited a California teen who had been shot in the stomach by a ghost gun in a 2019 school shooting while her best friend died of her wounds.

When I awoke on Nov. 14, 2019, my biggest worry of the day was a Spanish test, said Mia Tretta, speaking from the podium in a White House Rose Garden showing off rows of blooming tulips. We spent the morning like any other, laughing and talking. Until we heard a loud pop. It was a gunshot, followed by six more.

Biden had on hand a demonstration of a gun kit, walking over to pick up pieces that, within a half hour, can be assembled into a lethal weapon. Until now, guns made from kits like these lacked a serial number that would let police trace its ownership in the event of its use in a crime.