COPENHAGEN, Denmark -- The Danish inventor accused of torturing and killing Swedish journalist Kim Wall during a private submarine trip before dismembering her body is an intelligent man "with psychopathic tendencies," a prosecutor said as the murder trial opened Thursday. Peter Madsen went on trial Thursday accused of tying up and torturing Wall before he either cut her throat or strangled her during a trip on his private submarine in August.

Madsen, 47, is charged with murder, dismemberment and indecent handling of a corpse for the way he disposed of Wall's body.

Prosecutor Jakob Buch-Jepsen started the 12-day trial at Copenhagen City Court by reading the charges, describing in detail how Wall's body parts were found on the ocean bed. Madsen, wearing glasses and a dark shirt, listened quietly with his fists closed.

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Madsen's lawyer, Betaina Hald Engmark, then formally entered a not-guilty plea to the murder charge.

This photo allegedly shows Swedish journalist Kim Wall standing in the tower of the private submarine "UC3 Nautilus" on August 10, 2017 in Copenhagen Harbor. Getty

Testifying, Madsen repeated his claim that Wall died accidentally inside the UC3 Nautilus while he was on deck. He said Wall "had a wonderful evening until it ended in an accident," but denied that any sexual activity had taken place between them.

Madsen had offered shifting explanations for Wall's death prior to the trial. He initially told authorities he had dropped Wall off on an Copenhagen island several hours into their submarine trip. Then he said that Wall died accidentally inside the submarine when a hatch fell and hit her on the head.

On Thursday, he described how he found Wall lifeless after a sudden pressure problem in the submarine.

"I could not open the hatches. I heard Kim, it was not good," he said. He added that he tried to give her first aid when he finally reached her, but stopped because it was impossible to stay inside. "There was a risk of having a submarine with two deaths," he told the court.

The prosecution claims Wall's murder was premeditated because Madsen brought along tools he normally didn't take when sailing.

Wall's parents were also present Thursday at the trial.

Members of the court were shown a drawing of the multiple stabs to Wall's torso. An audio file of a radio exchange between Madsen and maritime officials from Aug. 11 - the day after Madsen and Wall embarked on their submarine trip- was also played. In the recording, Madsen said he had let Wall off on an island, and that there were no injured persons aboard but only technical problems.

The 33-ton, nearly 18-meter-long submarine sank south of Copenhagen shortly after being spotted afloat. Madsen reported "man overboard" over the radio and was then picked up alone.

Buch-Jepsen said that after Madsen was arrested on land, forensic experts found dried blood on Madsen's nose - "blood that eventually was proven to belong to Kim Wall."

Wall studied at Paris' Sorbonne university, the London School of Economics and Columbia University in New York, from where she graduated with a master's degree in journalism in 2013.

She wrote for The New York Times, The Guardian and other publications, reporting on topics such as tourism in post-earthquake Haiti and nuclear testing in the Marshall Islands.

Caterina Clerici, a friend from Columbia, said Wall had "a soft spot for misfits, for places and people that did not conform."

Madsen was a co-founder of Copenhagen Suborbitals, a private aerospace consortium to develop and construct manned spacecraft. In 2011, it launched a homemade 30-foot rocket eight five miles into the sky over the Baltic Sea, a step toward its unrealized goal of launching a person into space.

In an interview with Danish weekly Soendagsavisen in 2014, Madsen said he one day "hoped to have a criminal career," adding he didn't want to rob a bank because "no one must be hurt."

The submarine UC3 Nautilus of Danish inventor Peter Madsen is seen in Copenhagen, Denmark, March 7, 2018. AP

On the evening that he contacted Wall, Madsen also texted his associate Steen Lorck to call off a planned trip the following day in the submarine that first launched in 2008.

After Wall left to meet Madsen, her boyfriend received several text messages from her. He started worrying when the messages stopped coming and eventually alerted authorities, who launched a search for the submarine, which didn't have a satellite tracking system.

Investigators found dried blood inside the submarine, and divers eventually found Wall's body parts in plastic bags held down on the Baltic Sea bed by metal pieces. Her torso had been stabbed multiple times.

Police believe Madsen sank the submarine on purpose, and found videos of women being tortured and killed on his personal computer in his hangar. He did not make the videos himself, investigators said.

The trial at Copenhagen's City Court ends April. 25.