Story highlights A United Airlines flight bound for Beijing turned around five hours into flight

The FBI alerted the flight crew to a possible international parental kidnapping

A woman traveling with her son was detained upon their return

The 4-year-old boy was returned to his father

A Beijing-bound United Airlines flight returned to Washington's Dulles International Airport Thursday evening because of a child custody investigation.

United Airlines Flight 897 took off on time at 12:20 p.m. ET, but had to return to Dulles at the request of federal law enforcement, an airline spokesperson said.

The Federal Bureau of Investigation had received an alert that a mother on that flight was allegedly taking her child out of the country illegally, an FBI spokesman told CNN.

Upon the flight's return "the flight was met by law enforcement including FBI agents and three individuals were removed from the plane to include a grandmother, mother and child," FBI spokesman Andrew Ames confirmed, via email.

"The mother was taken into custody on suspicion of committing an international parental kidnapping and the child was reunited with the father. The grandmother was not detained."

The criminal complaint charges that Wenjing Liu, also known as Linda Liu, unlawfully attempted "to remove a child from the United States with intent to obstruct the lawful exercise of parental rights."

After separating and starting divorce proceedings in 2013, Liu and William J. Ruifrok III were awarded joint custody of their 4-year-old son, who was born in China and is a dual U.S.-Chinese citizen, the complaint stated. The 2014 custody agreement doesn't allow either party to travel outside of the United States without "express written and notarized consent of the other party, provided in advance of the trip," according to the complaint.

In an interview with law enforcement after being removed from the flight, Liu admitted violating the custody order by removing her son from the United States without his father's consent, according to the complaint.

She made her first appearance in federal court in the Eastern District of Virginia on Friday and has a bond hearing scheduled for Monday.

After the aircraft returned to Dulles, United assigned a new crew due to crew flight time limits, and the flight departed again for Beijing at 7:47 pm ET, the United spokesperson said.

CNN's initial attempts to reach a representative for Liu were not successful.

It's a federal crime in the United States for parents to take or attempt to take their children out of the United States or keep them out of the country to obstruct another parent's custodial rights. But while federal authorities may prosecute one parent, they can't necessarily return a child who is already outside the country to the other parent.

About one quarter of the 1,800 to 2,000 annual missing children reports received by the National Center for Missing and Exploited Children are for international parental abductions, according to Maureen Heads, a supervisor in the center's missing children division.

The U.S. Department of State tries to return kidnapped children through negotiation, sometimes made easier if the other country is also a signatory to the Hague Convention on the Civil Aspects of International Parental Child Abduction. The United States and more than 70 other countries have signed the convention, but it only applies if both countries involved are signatories.

"That leaves many countries where there is no partner" for the United States to negotiate with, said Heads. "It can be a real challenge at times for parents."