Naomi Lewis first told Prince William County police she didn't notice anything amiss in the hours before her 12-year-old son allegedly barged into the office at his middle school last month armed with a loaded rifle and ordered everyone onto the floor, a detective testified yesterday.

But the 38-year-old cafeteria worker later changed her story, saying she realized her son had brought a rifle and ammunition to school because she heard something rattling around in the back of her van when they arrived at Bull Run Middle School, according to testimony. When Lewis asked her son why he had put it there, he told her, "I don't know," Detective Stephen Piaskowski said. The Haymarket woman then locked the van and headed into work.

The testimony came yesterday at Lewis's preliminary hearing in Prince William General District Court on a felony charge of possession of a weapon on school property. Judge Frank B. Perry III ruled that there was enough evidence to send the case to a grand jury, which will meet Aug. 2.

The brief hearing -- the first substantial public court proceeding in the case -- did not shed light on a motive for what police say her son did June 18, the last day of the school year: attempt to carry out a plot to frighten or even kill his classmates.

Lewis's son, who is being held in the county juvenile detention center, is awaiting an Aug. 11 trial on numerous charges, including conspiracy to commit murder and conspiracy to commit abduction for money. A 13-year-old Haymarket boy, who police said helped plan the attack but backed out at the last minute, is scheduled to go to trial the same day on a charge of conspiracy to possess firearms on school property.

Under Virginia law, court proceedings in those cases will be closed to the public because the defendants are younger than 14.

Police have said Lewis's son sneaked out to the silver minivan about 8:30 a.m. to retrieve several weapons, using a car key his mother did not know he had. He then changed into full camouflage garb in a school bathroom. As the boy loaded a cartridge into the .30-06 rifle, police said, an assistant principal on routine rounds heard the sound of the gun being loaded.

The assistant principal, Jamie Addington, saw the boy in the restroom, called police and implemented the school's emergency plan, which was developed after the slayings at Colorado's Columbine High School in 1999, police said.

Police stormed the school and took the boy into custody, but not before the seventh-grader, who covered his face with a red bandana, burst into the main office with the loaded rifle and threatened students, employees and parents, police said.

Friends said the boy had been bullied and teased by classmates because he was slightly overweight and wore large glasses. The 12-year-old planned a "violent takeover" that would include taking hostages to extort money, and he had researched the Columbine High shootings, court documents say.

The seventh-grader brought two other guns -- a .410-gauge shotgun and a .22 caliber rifle -- and boxes of ammunition to the school, police officers testified yesterday. In addition, he carried cans of oil, lighter fluid and matches. Ralph Daigneau, a police crime-scene analyst, testified that the 12-year-old also had a camouflage pouch that held 84 cartridges for the rifle.

Police have said that the boy's mother was unaware of the plot and did not know that her son had a key to the family's minivan.

Piaskowski testified that he first interviewed Naomi Lewis a few hours after the incident, while they sat in the parking lot with another detective and one of Lewis's co-workers. Lewis, who was told of the allegations against her son, told him the morning had been "pretty much normal," Piaskowski said.

Piaskowski said he later learned that Lewis's son had told police a different story and confronted the boy's mother with that information. "She immediately apologized and said she was sorry," Piaskowski said.

If convicted, she would face a maximum sentence of five years in prison.

Naomi Lewis, flanked by a family friend and police, after a hearing in Prince William General District Court. Her son is accused of planning to kill people at his middle school.