Seconds after a motorist accidentally bumped him off his bicycle at a busy Adelphi intersection last fall, Alejandro Jose Grant calmly got up, pulled a small pistol from a backpack, walked over to the waiting driver and shot her in the head.

Several witnesses during the first day of Grant's trial yesterday on first-degree murder charges gave similarly chilling accounts of what they saw Oct. 8 when Joy Estrella Mariano Enriquez, a 19-year-old college student, was killed.

"He just shot her in the head. It was devastating. I couldn't believe it," testified Chinola Jenkins, who was riding in a car at the intersection of University Boulevard and Riggs Road shortly after 4 p.m., when the shooting occurred.

"She fell back. Her car just rolled into a pole," Jenkins testified.

Another witness, John T. Fowler, testified that Enriquez had stopped her car and was calling out to Grant, asking whether he was all right.

Wearing headphones, Grant sauntered over to Enriquez, screaming, "You don't know me!" He reached into her car and "shot her just like she was a cockroach. Then he turned and walked away," Fowler testified.

Grant, 27, of Silver Spring, faces life in prison without parole if convicted of the first-degree murder charge. At the time of the shooting, Grant was free on assault charges in the District and Montgomery County despite having failed three court-ordered drug tests.

In his opening statement yesterday, Grant's attorney, Clayton Aarons, a deputy public defender, told jurors in Prince George's County Circuit Court that the key to the case would be whether the slaying was planned.

"We're not standing here before you saying he didn't do it," Aarons said. For jurors to convict Grant of first-degree murder, Aarons told them, "the state must prove . . . that he planned this, that he had the intent {to shoot her} when he walked to her car."

Assistant State's Attorney Fran Longwell told the jury in her opening statement that numerous witnesses and physical evidence tie Grant to the crime.

"There's no mistake that this is the man who did it," Longwell said.

Witnesses yesterday gave a detailed account of what happened immediately after the shooting. According to testimony, Grant tried to ride his bicycle away but was followed by at least two motorists. Using cell phones, some witnesses called the police moments after the shooting, according to testimony.

They also described seeing Grant toss his bicycle into a trash bin and then run away.

Two Prince George's County police officers who responded to investigate the incident and who chased down Grant on foot testified that they saw him toss a black backpack into a circular trash bin.

Other officers testified that a shiny, silver-colored, brown-handled handgun was recovered from the backpack. They said the gun matched the description given by witnesses.

Prince George's County Police Cpl. Charles Hamby, one of the officers who caught Grant, testified that several witnesses identified him as the gunman within 10 minutes of the attack.

Longwell told jurors that a firearms examination showed that the gun recovered from the backpack was the weapon used to kill Enriquez. But, she said, the gun found that day was destroyed accidentally by Prince George's police.

Enriquez, of Wheaton, was a physical therapy student at Montgomery College and was on her way to class when she was killed. Yesterday, nearly three dozen relatives, friends and members of the local Philippine community (Enriquez was of Philippine descent) sat in the courtroom to watch the trial and comfort each other.

The day after the shooting, Prince George's County Police Chief John S. Farrell said Grant told detectives he shot Enriquez because "he felt she endangered him and he wanted her to know how it felt." Before he was bumped off his bicycle by Enriquez, Aarons told the jury, Grant had been knocked off his bike by a motorist previously that day. CAPTION: Alejandro Jose Grant, of Silver Spring, is charged in the death of Joy Estrella Mariano Enriquez, 19.