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Officials looking for Lori Vallow’s two missing children are asking anyone who visited Yellowstone National Park on Sept. 8 for their photos taken at the park.

Vallow's adopted son, Joshua Vallow, 7, and daughter Tylee Ryan, 17, have not been seen since September. The FBI said it had determined that the children, Vallow and an uncle, Alex Cox, were at the site Sept. 8.

The FBI and Rexburg police have asked anyone who visited the park to submit videos or images that may help in the search. The children, Vallow and Cox were in a silver Ford F-150 pickup with Arizona license plates.

The Idaho mother and her husband, Chad Daybell, have not been cooperating in the case, Rexburg police have said. On Thursday, Vallow was extradited from Hawaii to Idaho, where she is expected in court on Friday.

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"Law enforcement is seeking photos and video which may have captured images of these persons and the vehicle, or images of crowds and other park visitors where these individuals may be present," the FBI statement says.

The plates on the truck, a 2017 model, were identified as CPQUINT. The FBI said photos or video can be uploaded at fbi.gov/rexburg.

Vallow was arrested in Hawaii in February and charged with desertion and nonsupport of dependent children. Daybell has not been charged.

Alex Cox, who is Vallow's brother, died in December, The Associated Press reported.

In July, Cox shot and killed Vallow's former husband, Charles Vallow, after a confrontation. Lori Vallow and Cox were questioned by police but not charged at the time. Authorities are investigating the former husband's death.

Adding to the mystery, police have said that Joshua and Tylee's disappearances could be connected to the death investigation of Daybell's former wife, Tammy Daybell. The 49-year-old's death was initially ruled as natural after she was found dead in her home, but has since been classified as suspicious.

Tammy Daybell’s remains have been exhumed, but the autopsy report has not yet been released, the AP reported. Toxicology results on Cox also are not public yet.

Yellowstone National Park is mostly in Wyoming, but small parts of it extend into Montana and Idaho, and it is northeast of Rexburg. The national park had more than 4 million recreational visits in 2019, according to the National Park Service.