New York (CNN) -- Some cold cases see periodic bursts of activity and then go cold again.
The disappearance of Kara Kopetsky, a high school junior who vanished three years ago in Belton, Missouri, appears to be one of those cases.
This month, police conducted a grid search on a field five miles from Belton High School, where Kopetsky was last seen.
The search area was within the Kansas City limits, said Rhonda Beckford, Kopetsky's mother.
"Its near an old military air base. It's basically a wide open field," Beckford said.
She says police had searched the area before, and she doesn't know what prompted investigators to search again.
"When we searched the area before, it was not in the same way," explained Capt. Don Spears of the Belton Police Department. "This time we did a shoulder-to-shoulder grid search of the 400- acre area."
The April 7 search turned up no clues, and police say they're no closer to finding Kopetsky.
"She is a very beautiful girl, and so we often warned her to be careful, but like any teen, she had an attitude that she was invincible," said her stepfather, Jim Beckford.
Kopetsky was 17 when she walked out of her high school for the last time May 4, 2007. It was something she often did, her mother says. Kopetsky would leave around 10:30 a.m., return between 1 and 2 p.m. and stay until school let out at 3:30 p.m.
"She didn't get along well with two teachers who taught her mid-day classes, and so she would leave after her morning classes, take a break and then come back for her afternoon classes," Rhonda Beckford said.
That day, Kopetsky had forgotten one of her textbooks. She called home and asked her mom to drop it off at the school office. She also asked her mother to wash her uniform so she could work the 4 p.m. shift at Popeye's Chicken.
A school surveillance video shows Kopetsky walking down a corridor and out the door of the high school. But no one can say which way she went or whether she got into a car. It was the last time anyone saw or heard from her.
When Kopetsky didn't come home from school as usual, her mother and stepfather grew worried. They called police and reported her missing.
The worried parents say police told them that they believed Kopetsky was a runaway and that she'd come back on her own in a few days.
But Beckford is certain her daughter didn't run away. "I believe someone picked her up. She got into someone's car, someone she knew," she said.
Police say Kopetsky's cell phone records show that the last phone calls she made before leaving the school grounds include one to her mother and that she exchanged text and voicemail messages with her boyfriend for about 20 minutes.
But, police say, there was no activity on Kopetsky's cell phone after she walked out of school, indicating that the battery went dead or was removed from the phone.
In the days after Kopetsky was reported missing, investigators followed pings from the phone and conducted some searches in Belton but found no clues. Beyond that, they aren't commenting on the investigation.
Kopetsky's stepfather says the cell phone's long silence makes him suspicious.
"This doesn't make any sense," Jim Beckford said. "Kara was on her cell phone sending texts constantly. Her cell phone bill was typically 80 to 100 pages long."
Kopetsky's mother said her daughter's boyfriend was questioned, and his home and truck were searched. But police found nothing out of the ordinary.
Kopetsky left behind most of her belongings: money, clothes, an iPod and a new carton of cigarettes. Her bank debit card was left in her school locker and her bank account, with $150 from a recent paycheck, remains untouched.
According to Belton police, the case is being actively investigated. But with no certain evidence of foul play, police continue to characterize Kara's disappearance as an endangered and missing adult case.
The state of Missouri considers Kopetsky to be an adult because she was 17 when she disappeared.
Belton Police Capt. Don Spears said police are looking at several persons of interest but haven't narrowed their investigation to focus on a single suspect.
Kopetsky's family believes she was abducted by someone she knew, but police have not ruled out the possibility of abduction by a stranger or drifter.
"The school doesn't sit too far from a major highway, so it's not outside the realm of possibility," Spears said.
About a month after she disappeared, Kopetsky's case was eclipsed by the abduction and slaying of Kelsey Smith, who was snatched from a store parking lot in nearby Overland Park, Kansas. Smith's body was found in the Missouri woods, six miles from Kopetsky's home in Belton. A man was charged and pleaded guilty, and is serving a life sentence.
Police in Belton and Overland Park compared notes but found no connection.
Kopetsky is described as 5 feet, 5 inches tall and 125 pounds with brown hair and hazel eyes.
A $30,000 reward is offered for information leading to her whereabouts or the arrest of anyone responsible for her disappearance. Anyone with more information is asked to call the Belton Police Department's tip-line at 816-474-TIPS.