Strange, sad and macabre, the discovery of the skeletal remains of twin brothers Andrew and Anthony Johnson has mystified neighbours and others in Chattanooga and beyond.

Each man was found seated in an easy chair inside the modest home they shared for decades, and where they apparently died together about three years ago, with no obvious signs of foul play.

Even while they were alive, though, the 63-year-old twins were something of a mystery to their neighbours, who occasionally saw them wearing surgical masks while gardening but never saw them with visitors.

"I didn't even know their names," said retiree Linda Maffett, who lived across the street.

In an interview about the Johnsons she added, "It's a strange story, it's a sad story. I think it's sad that they were sitting there that long with nobody checking on them."

Police went to the home on 29 March, after being asked to check on the brothers by a relative who had a key. Officers found the twins' decomposing bodies sitting in recliners in the living room. Their conditions suggested that both men had been dead since 2011.

An autopsy helped confirm their identities, but preliminary results revealed no obvious signs of trauma or foul play, Chattanooga police spokesman Tim McFarland said. He said there was some flesh on the brothers' skeletal remains. The Hamilton County medical examiner is working on toxicology tests. In the meantime, McFarland said police are not speculating on a cause of death.

But Chattanooga residents are formulating their own theories. Was there a gas leak? Was it a double suicide? Were they poisoned? Or did they just sit down one day and die together?

Neighbours said they had not seen the brothers in at least a couple of years.

They said the Johnsons kept to themselves and didn't associate with others in the Hixson community. They might be seen working on their lawn or going for groceries together. But blinds blocked any view into the white house on Acorn Court in the quiet, hilly neighborhood of one- and two-story homes. The Johnsons' house remained dark, with no exterior lighting even at night, neighbours said.

Police made a welfare check on them in 2011 at the request of a relative, but found nothing untoward to lead them to break into the house. They said a relative told them that it would be unsurprising if the twins moved without telling anyone in the family.

Some just assumed the house was vacant.

Although the Johnson brothers had stopped cutting their own grass, neighbors said it kept getting cut. No one has been able to say who maintained it, whether a neighbourhood volunteer, relative or someone else.

A note inside the mailbox indicated that mail delivery had stopped because the postal service thought the Johnson brothers had moved.

Maffet, one of their nearest neighbors, said that years ago the short, stocky brothers could be seen at times working on their lawn while wearing surgical masks. It is not clear if they had health problems, though one day Maffett went to help one of the men whom she found lying on his front porch after he had fallen down. Maffett offered to get his brother but he said his sibling was deaf.

Maffett called 911 for assistance but never entered the house, and it was the only time she over spoke with either of them, she said.

"I never heard them say anything to each other," she said.

A Census worker even approached Maffett one day to ask about the whereabouts of the Johnsons.

Chattanooga police spokesman Tim McFarland. Photograph: Adrian Sainz/AP Photograph: Adrian Sainz/AP

Police say the brothers owned vehicles and had a valid driver's license. They had insurance and there were even records of them owning their own business. Investigators say there was no indication that the brothers suffered from mental illness. But they had little to no communication with others.

"While this incident is tragic and unfortunate, Andrew and Anthony lived a hermit lifestyle and did not communicate with family or anyone else," a police news release said.

The Johnsons' bodies were found just a few weeks after the mummified remains of a woman were found in the garage of a suburban Detroit home. Relatives said that while those remains have not been formally identified, they belong to homeowner Pia Farrenkopf, who had been missing for years while her bank automatically paid her bills.

Both her case and that of the Johnsons have prompted conversations about how well Americans keep in touch with their neighbours.

The Associated Press could not locate any relatives of the Johnsons. A man who was at the house Thursday ordered a reporter to get away from the property. He declined to identify himself.

The Johnson brothers were already living on Acorn Court when Maffett moved in about 18 years ago. Hamilton property records show the brothers bought the house in 1984 for about $43,000. Built in 1955, it is currently worth about $116,000.

Maffett said she felt something was wrong, but she did not check.

"I really felt bad that it happened and I didn't have the courage to go barging over there and knock on the door when I had that gut feeling," said Maffett, who is 73.

Cory Hall, 38, and his wife have been renting a home in the neighbourhood since December. Hall said he and other neighbours have talked about the deaths and they all are speculating about what happened.

"I would just like to know a little bit of the history about how they became such hermits," Hall said. "They had to be off everybody's radar for quite a while."