Savanary Uk, Luu's fiancee, testified that Luu and Siv had mutual friends and that, initially, everything was fine that night.
Later on, she said, Siv stood up and ordered everyone to sit.
"He was saying who knew about his brother's death," said Uk, 28, referring to the 2012 home-invasion slaying of Sophos "Sonny" Siv, 40.
Luu tried to calm Siv, telling him, "We're all friends, we're all brothers. Please don't do this," Uk said.
Siv then accused Luu of killing his brother, Uk said.
"He said, 'Where have you been? How come we haven't seen you around?' He said, 'You killed my brother.' . . . Then everything happened so fast. It was a blur," Uk said.
Siv pulled two guns, pointed them toward her and Luu, and pulled the trigger on one, but the weapon jammed, Uk said. The gunman then fired the second gun, striking Luu several times, she added, sobbing.
"He was down on the floor. I remember holding on to him. There was blood coming out of everywhere. I told him, 'Don't fall asleep.' "
Ol Khao, 39, a mechanic who worked with Luu, said he managed to disarm Siv after the shooting. "I kept talking to him. I said, 'Brother, you already killed the guy.' "
Defense attorney Fortunato Perri Jr. did not offer an argument on behalf of Siv following the testimony.
Assistant District Attorney Andrew Notaristefano said there is no evidence that Luu killed Siv's brother Sophos Siv, who was found beaten, bound and gagged with duct tape, and shot twice inside his Olney home in February 2012.
His slaying remains unsolved.
On Twitter: @MensahDean