It’s not hard to disappear in the rugged mountains of southwestern Montana, where authorities have spent four days hunting for an Army veteran accused of opening fire in a neighborhood bar.

The roads west of the small town of Anaconda in Deer Lodge County, where four people were killed in the shooting early Friday, feed into rough terrain filled with hunting cabins, hiking trails, tall trees and the state’s largest national forest.

“I can totally see how somebody could park a car and disappear,” said Erin Hansen, 45, a high school teacher who often hikes there.

The shooting suspect, Michael P. Brown, 45, grew up in the county, which has a population of about 10,000, and has lived there since returning about two decades ago, after his Army service. The authorities say he is most likely still armed.

Manhunts in high-profile cases are often resolved quickly, as hundreds or even thousands of law enforcement officers spread across a search area. But there are exceptions, particularly in dense forests or rural areas with lots of places to hide.

A military veteran accused of killing his three daughters in Washington State has been on the run for more than two months. And Eric Rudolph, the Olympic Park bomber, spent more than five years hiding in rural North Carolina before being caught while rummaging through a grocery store trash bin.

Officials are searching a large and heavily wooded area.

Police officers and federal agents are searching a wide expanse on foot, in vehicles and from helicopters. They’re also using thermal imaging technology to try to spot the suspect.

The search has concentrated on a mountainous area west of Anaconda. Stumptown Road, where officials say the suspect was last seen, sits between mountains covered in dense pine and aspen trees, whose peaks spend much of the year dusted with snow.

The Montana attorney general, Austin Knudsen, said that Mr. Brown stole a truck after the shooting and that he might be using the camping equipment and clothes he took from it to try to live in the wilderness. He might also be hiding in some of the many structures that dot the forests.

“We’ve got every cabin, every hunting site that’s known up there pinned,” Mr. Knudsen said.

Part of a national forest has been closed off for the search.

The authorities have not made public the exact borders of their search area, but it extends into Beaverhead-Deerlodge National Forest, Montana’s largest national forest. A small portion of the forest — about 22 square miles — has been closed off to aid in the manhunt.

The vast forest stretches for more than three million acres across eight counties in the southwestern part of the state.

Forest officials said on social media that the 22-square-mile section would remain closed until Aug. 17, unless the suspect is caught earlier and it can be reopened, giving a sense of how long officials might be preparing for the search to extend.

The region is dotted with copper mines, which once drove the economy.

Anaconda’s most distinguishing feature might be the brick smelter stack just outside town. At 585 feet, it is 30 feet taller than the Washington Monument. The stack is one of the last vestiges of the region’s once-thriving copper smelting industry.

But there are also old mine shafts that dot the mountains, fueling rumors throughout town that he might be hiding in one of those, or on the shores of one of the many lakes throughout the region.

Chris Fisk, a historian in the neighboring city of Butte, said it would be possible for someone to get into one of those old mines. But that was just one of many options for a desperate person trying to hide in the mountains.

The rough terrain could make it more difficult for the suspect, as well as for the search teams, Mr. Fisk said. That will be especially true once winter sets in — which isn’t as far off in Montana as in most other parts of the country.

“It can be inhospitable,” Mr. Fisk. “But if somebody wants to hide, they can do it.”