PINE RIDGE, S.D. (AP) – Authorities and volunteers are beginning their second week of searching for three missing men on South Dakota's Pine Ridge Indian Reservation.

Juan Lamont, 24, Tevin Tyon, 21, and Tyrell Wilson, 23, were last seen in Pine Ridge on May 7. The Oglala Sioux Tribe declared a state of emergency four days later, and authorities issued a multistate missing persons advisory.

The men told a friend they were going for a ride in the country, according to Stepheny Lamont, Juan's mother. Information from credit card usage and cellphone pings indicates they might have been heading to a reservation casino, Tribe Emergency Manager Marei Kingi said.

Authorities have speculated that they might have crashed or become stuck in mud in a rural area.

More than 200 volunteers – some from as far as Colorado – spent Sunday on the reservation searching for the men, using sport utility vehicles, horses and all-terrain vehicles. The search was continuing Monday.

“They vanished and we have no clue,” volunteer Eileen Janis told KOTA-TV.

Tevin Tyon had just returned to the reservation in April after a military tour of duty in Kuwait, mother Dorothy Tyon told KEVN-TV. She is now hoping for a second reunion.

“Our goal is to bring them home,” she said.

The men were traveling in a silver 2006 Buick Lacrosse with license plate F2893.