Matt Brown's brother was seen talking with police near the Washington river where an unidentified man disappeared as authorities continue searching for answers in the mystery surrounding the former "Alaskan Bush People" star's whereabouts ... TMZ has learned.

Sources familiar with the situation tell TMZ ... Gabe Brown was seen at the river location Thursday speaking with authorities. We're told law enforcement first spoke with Gabe on Wednesday as part of its ongoing investigation.

We broke the story ... sources told us last week Gabe is the only family member who occasionally still communicates with Matt, though we're told their contact is rare.

As TMZ previously reported ... a family member says they spotted Matt around 8 AM Wednesday near the Driscoll Island Wildlife Area off Highway 97 in Oroville, Washington -- less than a mile from where a witness later reported seeing a man swept away in the Okanogan River.

The relative told us the area is a campground and fishing spot Matt is known to frequent.

The sighting added a troubling new layer to the mystery after Matt's brother, Bear Brown, publicly expressed fears that Matt may be dead.

Despite a search by police, firefighters, and EMS personnel, the man has not been located. Authorities said Friday the search would resume when weather conditions improve.