A US man has been charged with second-degree murder over the death of a worker he was paying to build a nuclear bomb shelter underneath his Maryland home.

Key points: Askia Khafra died after fire broke out while he was working on the bomb shelter

Askia Khafra died after fire broke out while he was working on the bomb shelter Daniel Beckwitt prevented Mr Khafra knowing where the house was

Daniel Beckwitt prevented Mr Khafra knowing where the house was Charge of 'depraved heart' second-degree murder refers to acting with depraved indifference to human life

Montgomery County Police Department have charged 27-year-old Daniel Beckwitt with "depraved heart" second-degree murder and involuntary manslaughter over the death of 21-year-old Askia Khafra on September 10, 2017.

"Depraved heart" murder is a charge in US law where an individual is accused of acting with depraved indifference to human life.

Mr Khafra's body was found by firefighters in the basement of the stock trader's home after a fire broke out beneath the residence where Mr Khafra had been digging a network of tunnels.

Mr Beckwitt's lawyer Robert C Bonsib told the New York Times that his client had the shelter built because he was concerned about an increase in international tensions, particularly North Korea's capacity for firing long-range missiles.

According to the Washington Post, prosecutors and police described the tunnel network as starting at a 20-foot (six metre) drop from a hole in Mr Beckwitt's basement before branching out into a tunnel system totalling about 200 feet (60 metres) in length.

They claim the project needed a large amount of electricity, and was powered by a "haphazard daisy chain" of power cords.

Mr Beckwitt is accused in court papers filed in Montgomery County of being aware that a fire could break out, but took no steps to prevent it.

Prosecutors claim Mr Beckwitt was aware of smoke from the basement on the day of the fire but only reacted by attempting to adjust circuit breakers.

Daniel Beckwitt has been charged with second degree murder and involuntary manslaughter. ( Montgomery County Police )

Mr Beckwitt escaped the house on the day of the fire before alerting firefighters that another man was inside.

According to medical examiners, Mr Khafra died of smoke inhalation and "thermal injuries".

Mr Beckwitt went to extreme lengths to prevent Mr Khafra being aware of where the home was located.

Prosecutors claim he would pick Mr Khafra up from his home in Silver Spring, Maryland, and drive him across the state border to Virginia where he would be made to put on blackout glasses.

Mr Beckwitt would then drive to his house where he would lead Mr Khafra inside by string.

Only when Mr Khafra arrived at the entrance of the tunnel would he be allowed to take off the glasses.

Prosecutors claim Mr Beckwitt enticed Mr Khafra to work on the project by agreeing to invest in the 21-year-old's start-up venture platform for web-based investors.

But Mr Bonsib said Mr Khafra enjoyed working on the project and had posted photos of his progress on social media.

On Friday a Montgomery County judge ordered Mr Beckwitt to be released on US$100,000 ($132,000) bond.