A petrol-fueled explosion in a Copenhagen apartment on Sunday afternoon claimed the lives of a well-known restaurateur, her architect father, and her estranged, American husband.

Maj Kaltoft, 34, her father Ole Kaltoft, 69, and her husband Kirk Smith, 38, all died of extreme burns. Kaltoft was found dead on the scene, while her father, Ole Kaltoft, and estranged husband, Kirk Smith, died shortly afterwards at hospital.

The couple, who had recently split, is survived by their three-year-old daughter, who was not at the scene.

Investigators quickly determined that the explosion and fire were planned by Smith, an apparent murder-suicide.

According to relatives, Maj Kaltoft was going to the estranged couple’s apartment at Rahbeks Allé 5, to retrieve some belongings. She brought her father because she felt threatened, they said.

Witnesses confirmed that Maj and Ole Kaltoft arrived at the apartment building at 11:45am on Sunday. Less than 30 minutes later, the Copenhagen fire brigade was alerted about a loud explosion and fire.

Firemen found Maj KaltoftÂ’s severely burned body on the third floor landing. Her father Ole was found, barely alive, on the fifth floor. Smith was found, also barely alive, in the first-floor entryway. The two men died shortly afterwards at hospital.

Investigators found empty and full bottles of petrol strewn throughout the apartment. Video footage from a nearby petrol station showed Smith buying five litres of fuel in a spare tank, just an hour before the explosion.

The autopsies confirmed that all three were doused in petrol. However, there were no signs of physical struggle prior to the explosion.

Family members announced on Monday that Maj Kaltoft and Smith had recently suffered a family tragedy – the loss of their three-month-old son to sudden infant death syndrome (SIDS) eight months earlier.

Psychologists with expertise in SIDS, but no connection to the Kaltoft-Smith family, told Ekstra Bladet that severe depression was typical after the loss of a child, and often led to complications including divorce, job loss, and increased risk of death.

A shocked friend of the family told Ekstra Bladet that there was “nothing aggressive” about Smith.

“He was really caring, a really good man. It’s horrible that he now seems like such a monster. He wasn’t that at all.”

Maj Kaltoft was part owner with her brother, the chef Morten Kaltoft, of the four Famo restaurants in Copenhagen and Frederiksberg. Their father Ole Kaltoft designed Famo Metro, the newest of them.