RED LAKE, Minn. (AP) - Authorities called off major ground search operations on the Red Lake Indian Reservation on Sunday evening after they turned up no sign of two young brothers who have been missing since they went out to play Wednesday morning.

But FBI Special Agent Paul McCabe stressed that efforts to find 4-year-old Tristan White and 2-year-old Avery Stately are not over, and that investigators will continue to actively work the case.

"We will not stop looking for Tristan and Avery until they are found," McCabe said.

McCabe said there's still no firm evidence of what happened to the boys, leaving it unclear whether they wandered off or were taken.

"We don't have any indication at this time that foul play is involved, but we encourage people to continue to phone in tips if they have any information that might lead us to the whereabouts of Tristan and Avery," he said.

Investigators had received around 70 tips as of Sunday and agents are continuing to follow up on those, McCabe said, but added that none of these tips has led to the boys so far.

The FBI last week offered a $20,000 reward for information that leads to the boys.

The boys, both American Indian, have short brown hair and brown eyes. Tristan was described as 3-feet-6 and wearing a dark blue Spider-Man jacket with yellow trim, Levis jeans and black and gray winter boots. Avery was described as 2 feet tall and wearing a gray pullover sweat shirt that says "Timberland" on the front, faded Levis jeans and Spider-Man tennis shoes.

While all the possibilities for ground searches have been exhausted, authorities will conduct "limited and pinpoint searches" if new information surfaces, he said.