After admitting that he was a gang member and that he fatally stabbed a man last July, the youngest person ever prosecuted for murder in Fairfax County was found guilty yesterday of second-degree murder and immediately sent to the adult jail to await sentencing.

Jose Rodriguez was 14 when he faced off with 22-year-old Mario Rubio-Martinez in the parking lot of the Culmore Shopping Center on July 30. Rodriguez testified that Rubio-Martinez came toward him with a beer bottle, so he lunged at him once with his pocketknife. He said he did not know he had pierced Rubio-Martinez's heart.

"I had to defend myself or else I was going to get hit with a bottle in the head," Rodriguez said. "I did not mean to kill him."

Prosecutors sought a first-degree murder conviction -- which carries a sentence of 20 years to life in prison -- while defense attorneys argued for manslaughter. After more than five hours of deliberation, the jury decided on second-degree murder, with a sentencing range of five to 40 years.

Circuit Court Judge Henry E. Hudson will impose sentence April 27. He rejected a defense request that Rodriguez stay in juvenile detention until then. Defense attorneys hope Hudson will sentence Rodriguez to juvenile facilities until he is 21 so he can continue his education. Rodriguez turned 15 last month.

Police and prosecutors said in earlier hearings that Rodriguez had spent the night of July 29 hanging out with members of Mara Salvatrucha, a sometimes violent street gang with increasing tentacles through Northern Virginia. At a meeting in Fairfax City that night, prosecutors said, Rodriguez suggested he be disciplined by the gang for excessive drinking and smoking, and his comrades obliged with a beating.

After that meeting, Rodriguez wound up in a pickup truck with a young woman and 11 other young men, including his 12-year-old brother, Roberto, and his 13-year-old friend Cristobal Medrano. After a round of club-hopping, the three youths testified, their crowded pickup pulled into the shopping center lot about 3 a.m.

There, they encountered Rubio-Martinez and his brother Louis Martinez, who had just dropped off another brother and two women. The surviving Martinez brothers testified that they did nothing to provoke the gang.

Both Medrano and Roberto Rodriguez testified that the Martinez brothers were "flashing gang signs and disrespecting us, cussing at us," as Medrano said.

In a taped statement played for the jury, Jose Rodriguez told police several hours after the incident that the Martinez brothers "looked at me, right, and they had the bottle in their hands. So I was like, I thought they were going to hit me. So I just pulled out my knife and stabbed him."

Louis Martinez testified that his brother never had a bottle.

Rodriguez told the jury the same story Wednesday, adding that he feared for Medrano, his little brother and himself. He also said that Marvin Campos, 34, told him, "Don't do it," as he got out of the truck. Campos has pleaded no contest to participating in a gang crime with a juvenile, the first such person prosecuted under a new Virginia law. Prosecutors said he was a leader in the local Mara Salvatrucha gang.

Assistant Commonwealth's Attorney John R. Murphy told the jury that premeditation, a necessary element of first-degree murder, "can be formed in a second." He said the "only reason he did what he did was to show his friends he's a tough guy."

Alene C. Grabauskas, one of Rodriguez's lawyers, argued that Rodriguez's single swing at Rubio-Martinez "is not intent to kill."

"They are relying on the fact you will convict him based on his affiliation with the Mara Salvatrucha gang," Grabauskas said.