INDEPENDENCE, Ore. -- A fourth boat, with divers and cameras on board, joined dogs, helicopters and volunteers Monday in the search for two brothers who have been missing for more than a day.

The two boys, 20-year-old Alvin Troub and his younger brother, Michael Runyon, an 8-year-old with autism, were last seen Sunday fishing on the Willamette River banks in Independence.

Mary Troub, the boys’ mother, said Michael cannot swim and was probably “very, very scared.”

Search crews are focusing on the river, but family members aren't entirely convinced they fell in.

Authorities are focusing on the river, though, from the sky, ground and water.

Courtesy photo

Alvin Troub, 20, and his 8-year-old brother, Michael Runyon as shown in a family photo.

“I feel like they're out there. I feel like they're alive,” Mary Troub told KGW. “They're extremely close. Alvin's like a role model to Michael. He loves him to death.”

Authorities found the boys' fishing equipment yards away from the river, but no sign of them. The river was running very high, they said, and the boys could have fallen in.

“Our fear is that they're in the water,” said Independence Police Department Chief Vernon Wells.

More than two dozen volunteers are searching the murky water and thick brush along the banks -- but say the odds of finding the boys alive are dwindling by the hour.

“The assumption is they somehow got in the river and most of our search is concentrating on the river,” Wells said.

Troub isn’t so sure.

“I'm kind of thinking at this point they were taken,” she said.

"We just can't figure it out, [the boys' mom] was thinking someone may have taken the boys, but Alvin is such a big boy," said their aunt, Laurie Runyon. "Alvin takes really good care of his fishing poles. I don't know what's going on.”

The boys’ pictures are plastered on flyers in store-front windows from Independence all the way to Monmouth.

KGW photo

Michael Runyon as shown in a photo from his family.

Police reiterated they were working “every possible angle” in the case, including the possibility the boys may have been abducted.

At least one boat will resume searching for the boys Tuesday should darkness and weather conditions force a suspension of the hunt Monday night.