The 19-year-old German died early on Thursday while undergoing treatment at Dr. Kariadi hospital in Semarang.
Earlier, on Tuesday night, Lukas visited a bar at Pandanaran Hotel in Semarang, said Richo Vino Subangkit, his Indonesian friend.
"He had a few drinks at Pandanaran Hotel on Tuesday night. He even went for a swim the next morning, but his body convulsed on Wednesday midnight," he added.
After going into convulsions at Richo`s rented house on Pamularsih Street in Semarang, Lukar was admitted to the hospital.
However, the German was pronounced dead at 1.55 a.m. local time on Thursday while undergoing treatment at the hospital`s intensive care unit.
Taking into account that Lukas had a head injury and a bleeding nose, a doctor from the hospital reported the death to the police.
According to Gatot Sudharto, a forensic doctor from the hospital, Lukas likely died of a heart attack. However, the doctor said he was not sure what triggered the heart attack.
Meanwhile, Central Java provincial police chief Insp. Gen. Dwi Priyatno confirmed that Lukas had a head injury.
"There is a wound on his head, which suggests that he had a bad fall sometime before his death," he said.
However, Priyatno continued, the cause of Lukas` death can be ascertained only after further investigation.
Police are awaiting the permission of his family and the German embassy in Jakarta to conduct an autopsy, he added.
(Reporting by I.C.Senjaya/Uu.INE/KR-BSR/A014)