Searchers are using specially trained dogs and aircraft with sophisticated camera equipment. But concerns are growing as temperatures fall.
FBI spokesman Paul McCabe says the weather had been fairly mild until last night, when temperatures dropped into the teens. He says about 100 trained searchers and law enforcement officers were out looking this morning, and dive teams were searching three lakes close to the boys' home.
Four-year-old Tristan White and two-year-old Avery Stately were last seen playing outside their home Wednesday morning.
Family members are holding out hope, but also preparing for the worst.
As their grandfather put it, "We probably would have found them by now if they were in the area."
Dozens of trained searchers were taking to woods, lakes and air Friday to hunt for two young brothers who disappeared from an American Indian reservation two days earlier.
The FBI offered a $20,000 reward for information leading to the recovery of the boys.
They were reported missing Wednesday from the Walking Shield area of the remote, heavily wooded Red Lake Indian Reservation in northern Minnesota, the FBI said.
Their parents said they had been playing outside their home before they disappeared, Tribal Chairman Floyd "Buck" Jourdain Jr. said.
"They were out there one minute, and (then) they didn't hear them or see them," Jourdain said.
FBI spokesman Paul McCabe said it was too soon to tell if the boys wandered away or if a crime had been committed. "At this point, there's nothing to indicate either way," he said.
Temperatures, which fell to 25 degrees overnight, were expected to reach the mid-40s on Friday.
Since the boys disappeared, at least 300 volunteers have combed the forest on foot, horseback and four-wheelers. Small planes were to search overhead Friday, and a sheriff's dive team was planning to check lakes, although two days of ground searching had found no telltale breaks in the ice where the boys may have slipped in, McCabe said.
"The response from the Red Lake nation has been overwhelming," McCabe said.
Many volunteers put aside holiday plans to help Thursday.
"It's not been a very joyous Thanksgiving here," Jourdain said.
The disappearance came less than two years after 16-year-old Jeff Weise killed his grandfather and grandfather's girlfriend on the reservation March 21, 2005, then went to the high school and killed seven more people, including a teacher and a security guard, before killing himself.