Madyson was last seen riding her white Razor scooter, wearing a purple knee-high dress with black leggings, and a black a baseball-style helmet with long, dark hair pulled to the side in a braid.
The FBI has joined the effort in addition to the Santa Cruz County Sheriff’s Office Search and Rescue Team. It was not immediately clear if foul play was involved and the investigation was in the early stages, police said.
Specially trained K-9’s from the State Office of Emergency Services have helped track down Madyson’s scent in the city's main beach area about 2 miles away.
The search is expected to continue near the beach during daylight hours and include the San Lorenzo River waterway and levees, where watercraft, aircraft and foot teams will be utilized.
Madyson is described as being 4 feet tall, 48 pounds with long brown hair and blue eyes.