The search for a man accused of injuring five people when he opened fire on a Kentucky highway has stretched into its fifth day as authorities scour a "jungle"-like forest terrain.

The suspect, identified as 32-year-old Joseph Couch, shot at vehicles that were traveling on Interstate 75 from a cliff near exit 49 on Saturday evening, authorities said. Sheriff's deputies responded to calls at about 5:30 and found vehicles parked and riddled with bullets. Officials said at least a dozen vehicles were hit. Some of the five injured were in serious condition but all were expected to survive.

Couch planned to "kill a lot of people" and then himself, he said in a text message, according to an arrest affidavit. He had purchased a gun and ammunition hours before. He faces five counts of attempted murder and five counts of first-degree assault, and will likely receive more charges.

Area schools were closed for another day Wednesday and the community of London and Laurel County was still on edge as the search stretched on. Authorities say the Daniel Boone National Forest, near where the shooting unfolded, poses unique challenges to search crews because of its landscape and vastness.

MAP AND TIMELINE:After active shooter opens fire on Kentucky highway, massive manhunt continues

Why is the search taking so long? Daniel Boone National Forest is 'like a jungle'

Authorities have been scouring the rolling woods near Laurel County for days in search of Couch and other evidence linked to the shooting. The notoriously rugged terrain of the area has compounded the difficulty of finding him, according to law enforcement officials.

“We are in the Daniel Boone National Forest, and this is thousands and thousands of acres,” said Scottie Pennington, public affairs officer for Kentucky State Police, in a Monday news briefing. “It is like a jungle.”

Law enforcement officials have deployed helicopters, drones and dogs to search the area where Couch vanished into the forest, after officials found his car and gun near the scene of the shooting. Cold nights and humid days, paired with a lack of food and water, will hopefully draw him out of the woods to surrender himself to law enforcement, Pennington said.

Cliffs, sinkholes, caves, culverts, creeks and rivers, and dense brush are complicating the manhunt for the Laurel County shooting suspect, Pennington said, along with wildlife that includes venomous snakes.

“The Daniel Boone National Forest embraces some of the most rugged terrain west of the Appalachian Mountains,” according to the U.S. Forest Service, and contains “steep forested slopes, sandstone cliffs and narrow ravines.” 

Daniel Boone National Forest sprawls across more than 700,000 acres and 21 Kentucky counties. Taking a wrong step along wooded ridges or encountering dangerous wildlife adds risk in the region, especially off-trail, where law enforcement officials are now searching.

Daniel Boone National Forest has proven challenging to search teams in the past ‒ even in cases when the missing person wanted to be found. In July, search teams found Scott Hern, 48, alive after he went missing in the forest for two weeks and spent an estimated 12 days without access to food or water, according to search and rescue officials. 

'Please, please, please': 911 calls reveal panic, chaos on I-75

Victims of the I-75 shooting reported serious injuries and vehicle damage to 911 dispatchers, according to calls obtained by the Louisville Courier Journal, part of the USA TODAY Network.

One man told dispatchers he rushed to help a woman who had been shot in the hip.

"She's gushing blood, bad," he said.

Another victim can be heard moaning from the pain of a gunshot wound she said was in her leg.

"Please, please, please," she told a 911 dispatcher.

"An ambulance is on the way," the dispatcher from the London-Laurel County 911 Communications Center said. "They're coming."

Contributing: John Bacon, Cybele Mayes-Osterman and Minnah Arshad, USA TODAY; The Louisville Courier Journal