The abuse allegations against Brad Pitt relate to the treatment of one of his sons

Allegations that Brad Pitt was abusive on a private plane last week relate to the actor's treatment of his 15-year-old son, sources said Friday, as the FBI continued to gather information before determining whether to open an investigation.

FBI spokeswoman Laura Eimiller said the agency hadn't made a decision on a formal investigation into what occurred on a plane ferrying Pitt; his wife, Angelina Jolie Pitt; and their six children.

Several news outlets have reported that a child welfare agency in Los Angeles is investigating the well-being of the children, who range in ages from 8 to 15.

Sources familiar with the allegations but not authorized to speak publicly say the child welfare investigation centers on Pitt's conduct toward his son Maddox, 15, during an argument on the September 14 flight. No law-enforcement agency responded to the plane when it landed in Minnesota after the incident.

Amara Suarez, a spokeswoman for the Los Angeles County Department of Children and Family Services, said the agency could not confirm whether it was investigating Pitt or the well-being of the former couple's children.

Calls to the offices of Pitt's attorney, Lance Spiegel, and Jolie Pitt's lawyer, Laura Wasser, were forwarded to recorded messages stating their firms do not comment on clients.

Jolie Pitt filed for divorce Monday, and her lawyer released a statement the following day saying she came to the decision "for the health of the family." She listed their separation date as September 15, the day after the flight aboard the private plane, and the actress is seeking sole custody of all six of the children.

Perryn Hedlund, the sheriff of Minnesota's Koochiching County, told The Associated Press on Thursday that Brad Pitt was on a plane that landed at the International Falls, Minnesota, airport near the Canadian border on September 14.

Hedlund said his sheriff's deputies were not called to the airport, and the International Falls police were also not called.

"There's no incident whatsoever reported to law enforcement," Hedlund said.

He said he didn't know why the plane landed in International Falls but said it was not uncommon for hockey players or other celebrities to stop at the airport.

Pitt and Jolie Pitt — known as "Brangelina" — were together for 12 years but wed only in August 2014. They married privately at their French chateau in the Provence hamlet of Correns with their children serving as ring bearers and throwing flower petals. They announced the ceremony days later.

Their six children include 15-year-old Maddox, 12-year-old Pax, 11-year-old Zahara, 10-year-old Shiloh, and 8-year-old twins Knox and Vivienne.

This is the second marriage for Pitt, 52, who previously wed Jennifer Aniston. It's the third for Jolie Pitt, 41, who was previously married to Billy Bob Thornton and Jonny Lee Miller.