KALISPELL, Mont. -

Search crews were back out on Flathead Lake Tuesday, searching for any sign of Robert McDonald. The Polson doctor was last seen on Saturday when he went out on his kayak.

Crews are looking on the west side of Polson Bay, in a painstaking search that is now in its fourth day.

"It's broken into four zones right now, and what we're doing is we're using side-scan sonar and we're going back and forth in each of those zones," said Glenn Miller, the marine coordinator for the Lake County Sheriff's Office.

Miller captained the Lake County boat on Tuesday, and we went along with him on the search. We went line by line scanning the water with sonar, looking for any sign of an anomaly.

"We're looking for anything that doesn't appear to be natural, if it doesn't look like a rock," said Miller.

When they spot something on the sonar, they drop a buoy to mark the spot. Then they come back and scan the area again to see if it's anything worth diving down to check. Windy conditions on Tuesday were making it difficult for the boats to stay on course.

"It's hard to keep your tracks going and all that good stuff, so the wind doesn't help at all," said Miller.

The crews will continue to search until dark.

"The biggest thing we want is closure for the family. We want to know for sure whether he's there, get him back to his family," said Miller.

If nothing is found, search and rescue coordinators will regroup in the morning and re-evaluate their search plans to see if a new approach should be taken.