According to prosecutors, Madsen allegedly planned the murder before meeting up with Wall. He brought tools such as pipes, screwdrivers and a saw onto his home-built submarine so that he could torture the writer. These tools were used to hit, cut and stab Wall.
Though the cause of death has yet to be determined, prosecutors indicated Wall either died by strangulation or by having her throat slit.
Madsen, who was initially arrested in August 2017, was charged with murder; dismembering a corpse; sexual assault; endangering the lives, mobility and health of others; and deliberately sinking his submarine on January 16, the International Business Times reported.
The charge of endangering others' lives resulted from Madsen sailing into the routes of cruise and cargo ships around the time that the murder was committed.
Although the 46-year-old inventor admitted to dismembering and dumping Wall's remains in the ocean, he has repeatedly denied murdering or sexually assaulting the journalist.
The 30-year-old journalist was last seen on August 10, 2017, after the aerospace engineer took her out on the 56-foot submarine that he'd built. Wall boarded Madsen's submarine with the intention to get more information for a story that she was planning to write on the inventor.
Wall was reported missing by a friend on August 11, the day after she'd supposedly toured the submarine. Incidentally by this time, Madsen was already in police custody, as officials were investigating the case of his sunken sub. When asked about Wall's whereabouts, Madsen simply stated that he'd dropped her off on a beach a few hours before running into the troubles that ended with his submarine sitting at the bottom of the sea.
However, Madsen has repeatedly changed his account of what happened.
In one statement to police, Madsen claimed he'd opted for a sea burial for Wall after she'd died of carbon monoxide poisoning and in another statement he admitted that the journalist had actually died after a hatch fell on her head.
Suspicion around Madsen began to grow as officials realized that the sea burial Wall was given involved her body being cut up into pieces and bagged with pieces of metal so it would be weighed down to the seafloor.
Madsen, who will remain in police custody until his trial on March 8, faces a life sentence for the charges against him.