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.

The behind-the-scenes effort to find the boys has been "very intense ... and that will continue. That part of the investigation will not end until the boys are found," he said.

McCabe paid tribute to all of the hundreds of trained searchers and volunteers who have been out in the woods of the remote northern Minnesota reservation or searching by air and water almost around the clock since the boys were reported missing Wednesday. They included people from law enforcement agencies across the state as well as private citizens.

"Their efforts and dedication have been commendable," he said. "Many of them have been here since Thanksgiving Day. ... You can't say enough about how much we appreciate their efforts."

Falling temperatures over the last few days had dimmed hopes that the boys could have survived in the woods. By Sunday, afternoon temperatures in the area were in the 20s, and there was a chilly wind out of the northwest.

McCabe acknowledged that the searchers were disappointed with the decision to call off major ground operations.

"They've all been so focused and so dedicated to finding these two children that it's somewhat discouraging for them to have to leave," he said. Given that many had been searching for four or five days, he said, they were "emotionally vested. They really wanted to find these boys."