Meanwhile, the FBI filed a warrant giving it more legal authority to assist in the search.
The FBI's affidavit says Colleen Hauser left Minnesota on Tuesday with her son, Daniel, a 13-year-old who is refusing chemotherapy to treat his Hodgkin's lymphoma. The affidavit alleges Colleen fled the state to avoid being prosecuted on two state counts of depriving another of custodial or parental rights.
The FBI said the pair flew from Minnesota to Los Angeles on Sun Country Airlines. An airlines spokeswoman said information on passengers, method of payment, and travel companions are confidential.
Interpol, which coordinates police investigations across national borders, was joining the effort to find Colleen and Daniel Hauser.
"We're putting every effort we can into this," said Jason Seidl, a sheriff's deputy in southern Minnesota's Brown County.
The pair were last seen Tuesday morning in southern California, before authorities in Minnesota even realized they had left the state. Investigators suspect they might have headed to one of a number of alternative cancer clinics in northern Mexico.
Doctors say Daniel has a cancerous tumor growing in his chest that's likely to kill him without chemotherapy, but the Hausers prefer natural healing methods inspired by American Indian traditions.
The American Cancer Society estimates there are 35 to 50 clinics in Mexican border towns that attract cancer patients looking for alternatives to traditional U.S. treatment methods. Many of these clinics have offices in the San Diego area that serve as contact points for U.S. patients, who are then referred to clinics in Mexico.
On Thursday, Anthony Hauser appeared before reporters asking his wife to call him and to come home. "If you're out there, please bring Danny home so we can decide as a family what Danny's treatment should be," he said.
Anthony Hauser did not return phone calls seeking comment on Friday.
The Brown County charges of depriving another of custodial or parental rights are listed in a felony arrest warrant that would ease extradition if Colleen Hauser is caught in the United States. The "parental rights" listed in the charges are those of Brown County family services, which has been granted custody of Daniel to get him to a pediatric oncologist.
The federal paperwork filed Friday gives the FBI jurisdiction to assist in finding and arresting Hauser on those state charges.
Authorities have said their options are less clear if the pair is already in Mexico.
At a news conference Thursday, Brown County Sheriff Rich Hoffmann vowed to arrange a safe return for Colleen Hauser without an enforcement action if she shows "a good faith effort to come back."
Philip Elbert, the court-appointed attorney for Daniel, said Friday he hadn't heard from his client.
On Thursday, Elbert said he believed the family still had a chance to get their wish if they return.
Saying the judge in the case wants additional testimony, Elbert told CBS' The Early Show Thursday that Daniel will have a voice if he appears in court.
"Right now, nothing is final and Daniel needs to be heard," Elbert said.