MIDLAND CITY, Ala. -- The standoff between police and a gunman accused of holding a 5-year-old boy hostage in an underground bunker dragged into a fourth day on Friday, as authorities sought to continue delicate conversations with the man through a pipe and worked to safely end the tense situation.

Police said Jimmy Lee Dykes shot a school bus driver to death, grabbed the child off the bus and slipped into an underground bunker on his property in rural Alabama, where the he and the boy have been since Tuesday. There were signs the standoff could go on for some time: the shelter has electricity, food, TV, and police have delivered the boy's medication through a ventilation pipe leading to the bunker.

Hostage negotiators have used the pipe to talk to the gunman, but investigators have been tightlipped about their conversations.

Former FBI hostage negotiator Clint Van Zandt said authorities at the scene shouldn't rush to resolve the standoff as long as they are confident that the boy is unharmed. He cautioned against any drastic measures, such as cutting the electricity or putting sleep gas inside the bunker because it could agitate Dykes.

The negotiator should try to ease Dykes' anxieties over what will happen when the standoff ends, and refer to both the boy and Dykes by their first names to humanize them.

"I want to give him a reason to come out," Van Zandt said, "and my reason is, 'You didn't mean that to happen. It was unintentional. It could have happened to anyone. It was an accident. People have accidents, Jimmy Lee. It's not that big a thing. You and I can work that out."'

The shelter was about 4 feet (1.2 metres) underground, with about 6-by-8 feet (1.8-by-2.4 metres) of floor space and the PVC pipe that negotiators were speaking through, said James Arrington, police chief of the neighbouring town of Pinckard.

"He will have to give up sooner or later because (authorities) are not leaving," Arrington said. "It's pretty small, but he's been known to stay in there eight days."

Republican Rep. Steve Clouse, who represents the Midland City area, said he visited the boy's mother Thursday and that she is "hanging on by a thread."

Clouse said the mother told him that the boy has Asperger's syndrome, an autism-like disorder, as well as attention deficit hyperactivity disorder, or ADHD. Police have been delivering medication to him through the pipe, he added.

The red clay road leading to the bunker teemed Friday with more than a dozen police cars and trucks, a fire truck, a helicopter, officers from multiple agencies and news media near Midland City, population 2,300.

Police vehicles have come and gone steadily for hours from the command post, a small church taken over for that use.

Early Friday, activity picked up when a team in military-style uniforms, many toting weapons, got out of a big van in the pre-dawn chill and moved into a staging area. One appeared to be dog handler.

Dykes was known around the neighbourhood as a menacing figure who neighbours said once beat a dog to death with a lead pipe, threatened to shoot children for setting foot on his property and patrolled his yard at night with a flashlight and a firearm.

The chief confirmed that Dykes held anti-government views, as described by multiple neighbours: "He's against the government -- starting with Obama on down."

No motive has been discussed by investigators, but the police chief said the FBI had evidence suggesting it could be considered a hate crime. Federal authorities have not released any details about the standoff or the investigation. The mayor said he hasn't seen anything tying together Dykes' anti-government views and the allegations against him.

Authorities said the gunman boarded a stopped school bus filled with children on Tuesday afternoon and demanded two boys between 6 and 8 years old. When the driver tried to block his way, the gunman shot him several times and took the 5-year-old boy.

The bus driver, Charles Albert Poland Jr., 66, was hailed by locals as a hero.

Dykes had been scheduled to appear in court Wednesday to answer charges he shot at his neighbours in a dispute last month over a speed bump. Neighbor Claudia Davis said he yelled and fired shots at her, her son and her baby grandson over damage Dykes claimed their pickup truck did to a makeshift speed bump in the dirt road. No one was hurt.

The son, James Davis Jr., believes Tuesday's shooting was connected to the court date. "I believe he thought I was going to be in court and he was going to get more charges than the menacing, which he deserved, and he had a bunch of stuff to hide and that's why he did it."