Dozens of trained searchers, including those from the FBI, continued looking in the woods, lakes and air Friday for two young boys who went missing from the Red Lake Indian Reservation in northern Minnesota.

Tristan Anthony White, 4, and Avery Lee Stately, 2, disappeared from a yard in the Walking Shield area of the city of Red Lake between 9:30 a.m. and 9:50 a.m. Wednesday, the FBI said.

The Minneapolis office of the FBI is now offering a $20,000 reward for information leading to the recovery of the two brothers.

The boys' relatives made an appearance late Friday afternoon asking for the public's help after a day-long search came up empty.

Authorities are increasingly concerned for the boys, since colder weather is expected in the region this weekend. It was partly cloudy and in the mid-30s Friday morning in the area. The National Weather Service was predicting decreasing temperatures during the day and a 30 percent chance of rain or snow.

A dive team from the St. Louis County Sheriff's Office was to check lakes near the woods where the boys disappeared, although FBI Special Agent Paul McCabe said two days of ground searching found no tell-tale breaks in the ice where the boys may have slipped in.

Fixed wing aircraft were scheduled to search overhead, while federal, state and local law enforcement officers were on the ground. A rapid-response team of retired law enforcement officers from the National Center for Missing and Exploited Children arrived Thursday, McCabe said.

While the search started Friday with professionals, more volunteers would be requested later in the day, he said. Since the boys disappeared, at least 300 hundred volunteers on foot, horseback and four-wheelers have combed the surrounding forest.

Searchers were also going door-to-door and looked through houses in the neighborhood where the boys went missing.

Investigators were checking the possibility that the brothers were abducted.

McCabe said it was too soon to tell if the boys wandered away or if a crime had been committed on the remote reservation north of Bemidji. "At this point, there's nothing to indicated either way," he said. "We are pursuing a two-pronged approach."

The reservation differs from most reservations around the state in that the FBI has primary law enforcement jurisdiction there.

Hundreds of people, including volunteers and law enforcement officers, searched the area on Wednesday and Thursday. "The support from the Red Lake nation has been overwhelming," McCabe said.

Among them were Al Downwind and his children Danielle, 11, Alvin, 13, and Brandon, 7. "We felt the need to help," Al Downwind said. "All I know is that there are two young boys missing."

Tribal Chairman Floyd (Buck) Jourdain Jr. said the search area is mostly composed of wetlands, lakes and woods. He said no tracks or broken ice had been found to suggest the boys went into the water.

"All we really have to go on is the parents, who said the children were playing outside the home," he said. "They were out there one minute, and they didn't hear them or see them outside the house."

As darkness fell after a futile day of searching Thursday, he said: "It's not been a very joyous Thanksgiving here. Many of those people didn't eat dinner today. They've been out looking for those little 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.

Jourdain said that the children were "just out playing in front, and the family called for them, and there was no response," according to the Minneapolis Star Tribune. When a quick search through neighboring yards turned up nothing, family members called police, who arrived and began their search within 45 minutes. Officers also searched about 30 houses in the area, including crawl spaces.

The latest turmoil over the reservation's children comes less than two years after 16-year-old Jeff Weise killed his grandfather and grandfather's girlfriend on the reservation on March 21, 2005, then went to the high school school and killed seven more people, including a teacher and a security guard, before killing himself.

Anyone with information can call the FBI at (612) 376-3200 or the Red Lake Tribal Police Department at (218) 679-3313.

The Associated Press contributed to this report.