A Chinese girl who was caught being smuggled into the country and was sent to a Fairfax County foster home disappeared this week after stepping off a school bus, police said yesterday.

Officials say one theory is that she may have been abducted by the same group that brought her from Asia, but they said it also was possible that she had run away or that she had left with a relative.

FBI and immigration agents yesterday joined Fairfax police in looking for Wang Yuanxia. Officials said she is 12 years old, but her foster parents estimated her age as 14 or 15.

Wang was one of several Chinese nationals who entered the United States illegally at Dulles International Airport in the past six months, authorities said. In some of those cases, officials think Asian smugglers known as "snakeheads" were involved -- individuals who deliver illegal immigrants to restaurants or brothels where they are held in virtual servitude to pay off huge smuggling fees.

Law enforcement officials said several of the Dulles cases involved high-quality forged Japanese or Thai passports and may be related. Wang, however, used a real Thai passport that had been altered, so her case appears to be different. Why she was smuggled here remained a mystery, officials said.

In an interview yesterday, Robert Conway, her foster father, described the girl's disappearance as a "nightmare."

Incha, as the family knew her, had lived in their Lincolnia area home for six weeks. "I had been warned that the Chinese mafia might be looking for her," Conway said.

Klaharn Chaichana, a Thai citizen, brought Wang to Northern Virginia on a United Airlines flight from Paris on Aug. 25. He told immigration officials at the airport that the child was "his niece and a manager in the silk area of his business," according to documents filed in U.S. District Court in Alexandria.

However, the girl was unable to communicate with two Thai translators at the airport, according to authorities. Eventually, officials summoned a Chinese translator and Wang said she was from Fuzhou, the capital of Fujian province in southern China, which INS officials consider a major source of human smuggling. Wang said that she had met Klaharn seven days earlier and that her parents had paid to have her brought to America, according to court records and law enforcement officials.

A law enforcement official close to the case said it appeared that Wang's parents had paid a significant amount of money. "It doesn't look like a slavery case or an extortion case," the official said.

Klaharn was arrested and he subsequently pleaded guilty to aiding and abetting the use of an altered passport. He remains in custody. Klaharn told federal officials that a man named Chan Chai had given him airline tickets to Brussels and money for train and plane tickets from there to Paris and the United States. He said he met Wang in Bangkok, traveled with her and was supposed to deliver her to the Holiday Inn at 15th Street and Rhode Island Avenue NW in the District, according to court documents.

Fairfax police said that they were looking into possible connections between Wang's abduction and her illegal entry. "But we have nothing to confirm that there's a relationship," police spokesman Warren Carmichael said.

Klaharn is the fourth person arrested at Dulles in less than six months for attempting to bring in Chinese nationals, according to federal court records. In one other case, one of the people being smuggled was younger than 18.

In September, an Alexandria Juvenile and Domestic Relations Court judge ordered Wang into foster care until officials decided what to do with her. She could be deported but also could be permitted to stay in U.S. foster care, said her attorney, Ken Labowitz.

Wang told her foster parents she had argued with her parents in China after they withdrew her from school. This summer, her parents told their only child that she would be going to America to live with an aunt.

On Sept. 8, Virginia social workers contacted Robert and Caroline Conway, who have cared for three other children, and they agreed to take Wang. The girl spoke no English, so Caroline Conway downloaded a phonetic list of Chinese words and phrases from the Internet.

For the first three weeks, Wang stayed with a Chinese-speaking babysitter during the day. At night, she chopped vegetables and shared meals with her new family. She often watched an Asian cable television channel.

"She's very smart, very quick," Caroline Conway said. Wang told her foster parents she was 12, but they suspected she is two to three years older. She is 5-foot-6 and weighs 118 pounds.

Wang enrolled at Holmes Middle School on Sept. 28 and was the only Mandarin-speaking student; one teacher who also knew Mandarin would check on her. Wang and one of her foster sisters typically returned home on a bus about 3:10 p.m. But on Wednesday, Wang took the bus home alone. She was not at home when a cab arrived at 3:30 p.m. to take her to a scheduled doctor's appointment, the Conways said.

Although the couple said Wang seemed to like her new family, they couldn't tell if she might be inclined to run away.

"Because of the language barrier, we didn't know exactly how she felt inside," Caroline Conway said. "She couldn't confide in us."

The girl's attorney, Labowitz, said he is concerned.

"She had no other place to go and she is a long way from home with no possible way to get back," he said. "My fear is we will never know what happened to this child. It's very chilling." Staff writer Sylvia Moreno contributed to this report. CAPTION: WANG YUANXIA ec