FBI spokeswoman Lindsay Godwin said Johnson has been with the agency for six years and was recently transferred from the Newark, N.J., field office to D.C. to provide protection for the FBI director or the U.S. attorney general.
"It's a tragedy, and our hearts go out to the families of everyone involved because it's been terrible," Godwin said.
Prince George's County police said Johnson appeared to be at fault in a two-car accident that killed 18-year-old Lawrence Garner Jr. in Brandywine on Monday night.
Alcohol and speeding appeared to have been factors, police said.
Johnson, who was off duty and driving his personal vehicle at the time, has not been charged.
Law enforcement officials in Prince George's County said charges in fatal accident crashes are generally filed after the crash investigation is completed, which could take about three to six months.
The case will be handed over to the state's attorneys office for review, and the FBI agent could face vehicular manslaughter charges, police said.
Johnson had not yet begun his new protective detail and was still in training, Godwin said.
Police said Johnson was driving north on the 10600 block of North Keys Drive in Brandywine around 10 p.m. Monday when his 2002 Mitsubishi Montero crossed into oncoming traffic and struck a 2002 Hyundai Sonata head on.
Garner, who lived in the 8700 block of Sheehan Drive, was taken to a hospital with a 19-year-old passenger. Garner died and his passenger remains in critical condition.
Garner had graduated from Gwynn Park High School and worked in the lumber department at Home Depot in Upper Marlboro.