As his best friend pulled onto North Keys Road in Brandywine, Robert Mitchell II saw headlights coming toward him, apparently on the wrong side of the road. The next thing he remembers, Mitchell testified Monday, was the hospital.

What led to those traumatic moments — an automobile crash that left Mitchell, then 19, critically injured and 18-year-old Lawrence Garner Jr. dead — will be dissected this week in Prince George’s County Circuit Court, as prosecutors and defense attorneys haggle over who bears responsibility. Former FBI agent Adrian Johnson is charged with vehicular manslaughter and other offenses in the February 2011 crash.

Prosecutors have accused Johnson of drinking and driving erratically before the collision with Garner’s car. Defense attorneys say that while that might be true, Garner’s actions — pulling onto a rural road in the middle of the night — are to blame.

Mitchell testified briefly on the first day of Johnson’s trial. He told jurors that he and Garner had just played basketball and were headed home from another friend’s house when they pulled onto North Keys about 10 p.m. He said that he and Garner looked both ways, and saw nothing, before pulling out.

“After that it was just a bright set of lights,” said Mitchell, now 21. “They were on our side of the road.”

Prince George’s Assistant State’s Attorney Sam Danai said in his opening statement that Johnson had been drinking before the crash, and witnesses saw him driving erratically on North Keys.

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

“He was speeding, he was driving recklessly, and — more important than those two factors collectively — he was driving drunk,” Danai told jurors.

Robert Bonsib, Johnson’s defense attorney, said that although witnesses might testify about Johnson’s actions before the crash, jurors should focus on who caused the collision. He said Garner pulled in front of Johnson’s sport-utility vehicle “at the top of a hill . . . in the middle of the night,” and that Johnson had the right of way. He said Johnson veered away to try to avoid Garner’s car.

“What is at issue is what caused the accident, at the time of the accident, at the place of the accident,” Bonsib said. “It’s not enough to be drinking. It’s not enough to be speeding. It’s not enough to be other things if those things don’t cause the accident.”

Additional testimony in the case is scheduled for Tuesday; the trial is expected to go all week.