A 13-year-old girl may have been picked up Tuesday by her grandmother before she was reported missing from her Montgomery home, police said.

Police are still searching for Kiah Miller, who was last seen Tuesday morning wearing a blue backpack, a red long sleeve hoodie and black workout pants with red stripes. An Amber Alert has been issued in her disappearance.

The search has now turned to Oklahoma after the child's mother said the grandmother may have taken the child there, Montgomery Police Chief Anthony Solomon said. The grandmother is from Oklahoma, and text messages discovered by police show the child talking to the grandmother right up to the time she left, Solomon said.

The girl was reportedly staying with her father, who has sole custody and reported her missing around 8:45 a.m. Surveillance footage from a neighbor's house showed a car that kept passing by the home around 4:45 a.m., police said.

Solomon said his department has been coordinating with Oklahoma state police and the FBI. Her disappearance is still considered a kidnapping, as she may have been taken across state lines while her father has custody, Solomon said.

"Not saying it was against her will but there is a court order that is in place," Solomon said.

The Amber Alert indicated police are looking for Leola Morris in connection with her disappearance, as well as a 2012 Nissan Altima with an Oklahoma license plate. Solomon did not say whether Morris was the grandmother, or whether the car was the one seen on surveillance images.

Morris reportedly has one leg. Law enforcement believe the child could be in grave or immediate danger, according to the amber alert.

Julian Gill is a digital reporter in Houston. Read him on our breaking news site, Chron.com, and on our subscriber site, houstonchronicle.com. | julian.gill@chron.com | NEWS WHEN YOU NEED IT: Text CHRON to 77453 to receive breaking news alerts by text message | Sign up for breaking news alerts delivered to your email here.