The search widened Saturday for two young brothers missing from the Red Lake Indian Reservation, with investigators concerned about a night of cold temperatures and the growing time the boys have been gone.

Temperatures dropped into the teens on the northern Minnesota reservation Friday night and early Saturday, the coldest yet since Tristan Anthony White, 4, and Avery Lee Stately, 2, went missing Wednesday morning.

"The weather had been mild, but last night with those kinds of temps that does concern us, if there were children in the woods going into a third night," FBI Special Agent Paul McCabe said Saturday morning.

McCabe said that teams of searchers with specially trained dogs as well as aircraft with sophisticated camera equipment continued to search overnight Friday into Saturday morning. The FBI was also continuing its investigation into the possibility that the boys were taken; McCabe said a tip line set up Friday generated about 35 phone calls but no hard leads.

Members of the boys' family were holding out hope late Friday that they would be found, but were also preparing for the worst.

"We probably would have found them by now if they were in the area," said their grandfather, Myron Jones.

McCabe said there were about 100 trained searchers and law enforcement officers out searching Saturday morning.

He said dozens of volunteer searchers continued to offer their help, and were being paired up with trained searchers.

Teams were re-checking areas already searched once, McCabe said, as well as branching out further into the woods along cleared paths that the boys might have wandered down.

He said aerial searching would continue by both piloted planes as well as unmanned aerial surveillance vehicles. Dive teams from St. Louis and Cass counties were a part of the efforts, using underwater cameras and concentrating on three lakes close to the boys' home.

McCabe said the FBI is continuing its two-pronged approach, and still has no evidence to indicate if the boys wandered off or were taken.

The disappearances appeared to be causing jitters in some other Minnesota communities. On Friday night, police in St. Cloud nearly 200 miles south sealed off a Walmart store for a brief time after getting a call that a man who had a small boy with him was heard in the store commenting that he needed to "get back to Red Lake," according to the police report. Officers quickly located the man and boy in the store and confirmed both their identities.

The FBI is offering a $20,000 reward for information that leads to the recovery of the boys.

McCabe said the FBI is assessing day to day how long it continues the extensive ground search.

"At some point when you look at all the factors and statistics we'll have to end the ground search if it's not successful," McCabe said. "We haven't reached that determination yet."