The former FBI agent charged in connection with a car crash that killed one teenager and seriously injured another in Brandywine last year was convicted of motor vehicle manslaughter and six related charges Thursday.

As a jury foreman repeated “We find the defendant guilty” for each count, family members of 18-year-old Lawrence Garner Jr. gathered in Prince George’s County Circuit Court began to sob and hug one another. When a judge ordered the former agent, Adrian Johnson, jailed until his Dec. 14 sentencing, a woman in the front row smacked her hands together and said, “Yes!”

Prosecutors had accused Johnson, 38, of speeding and driving drunk when he plowed into the side of a car as it pulled onto North Keys Road in February 2011. The crash killed Garner, the driver, and seriously injured his best friend, Robert Mitchell II, as they returned home from a friend’s house after playing basketball.

“My family’s gone through a lot, and it feels like a sense of peace,” said Charlane Garner, Lawrence Garner’s mother. “I truly miss my son. He was the baby.”

Robert Bonsib, Johnson’s attorney, said he was “disappointed in the outcome” and planned to appeal. He said evidence of Johnson drinking made it “a very difficult case for us” but did not prove Johnson was responsible for the crash.

Jurors deliberated for just more than two hours. Johnson faces a maximum of 13 years in prison, according to State’s Attorney Angela Alsobrooks, although state sentencing guidelines call for a term of three months to four years.

In closing arguments Thursday, Prince George’s Assistant State’s Attorney Sam Danai told jurors that the two teens took all the precautions they could have, each looking both ways before pulling onto North Keys Road. Had Johnson not been drinking and speeding, Danai argued, he might not have veered into their lane and struck their car.

Danai said witnesses saw Johnson speeding and weaving around other vehicles before the crash; afterward, his blood alcohol concentration registered as .25 on a breath test.

“What do all of these amount to?” Danai asked jurors. “Gross negligence.”

At the time, Johnson was an FBI agent assigned to the state attorney general’s protection detail. He was off duty when the crash occurred and resigned shortly after.

In his closing argument, Bonsib acknowledged that Johnson had been drinking and said he might have been speeding, but he argued that neither of those factors caused the crash. He said Johnson was not impaired by alcohol, and Garner’s pulling in front of him in the middle of the night — on a road where Johnson had the right of way — was to blame.

“I don’t mean to be heartless or uncaring here, but I’ve got to say it: Mr. Garner was responsible for his death and the injuries to his passenger,” Bonsib said.

A defense expert testified that the collision would have occurred whether Johnson was speeding at 58 mph — as a crash investigator testified — or driving legally at 40 mph.

Bonsib said the testimony proved the crash was “inevitable” and rendered irrelevant any unflattering testimony about Johnson’s driving before the crash. Danai countered that the expert had been paid $9,000 to testify and said his analysis “defies logic.”