The sister of a student who stabbed an academic to death six days after knife charges against him were dropped has told an inquest she had previously warned police her brother was mentally unwell.

Femi Nandap, then 23, killed Jeroen Ensink, a stranger to him, seven months after being arrested for behaving strangely in the street and being in possession of a large knife.

Ensink’s widow has called on his inquest to discover why Nandap, from Nigeria, who was suffering from severe psychotic illness, was allowed to be at liberty and armed when he killed her husband, a senior lecturer at the London School of Hygiene and Tropical Medicine, in December 2015.

Dr Jeroen Ensink. Photograph: James Drew Turner/The Guardian

Nandap’s sister, Wupya Nandap, told St Pancras coroner’s court she repeatedly rang police in May 2015 after witnesses reported seeing him with a knife in a north London street, leading to him being arrested after a violent struggle with officers at her flat.

She had been concerned over her brother’s deteriorating mental health for four or five months, she said. He was withdrawn, had poor personal hygiene and was obsessed with conspiracy theories about the Bible, historical maps, giants, telepathy, aliens “and lots of things”.

He was also paranoid and worried that people were going to hurt him or his family, she said. She was not sure how much of this she had told police after his arrest, but she said she told them she was worried about his mental health.

“I said to [police] I didn’t think he was mentally well, and that he shouldn’t be questioned without a responsible adult and that I was happy to be that responsible adult,” said his sister, who is a lawyer.

“They said he had been assessed by a doctor, who said he was fit to be questioned alone. I asked if I could speak to him and they said I would have to call back in the morning.”

She said police asked her if Nandap might have taken illegal substances. “I said yes. He had been smoking a lot of cannabis and I didn’t think he should be questioned alone.” She said she must have called the police at least 10 times that night and managed to speak to them on four occasions.

The inquest has heard that police noted her concerns but did not create a Merlin report, which would have highlighted his mental illness to other agencies. Nandap was then released on bail and immediately flew home to Nigeria. He returned to the UK in October 2015.

Charges against him for the May incident were dropped by prosecutors on 23 December 2015. Six days later Ensink, 41, who was posting cards to friends announcing the birth of his daughter Fleur, was killed in the random attack near his Islington home.

Nandap’s sister said that while her brother was in Nigeria he saw “doctors, psychiatrists, so-called healers”. They diagnosed psychosis, acute depression and auditory hallucinations.

She said he was placed on medication, and in October felt better and wanted to resume his studies in London. She said he seemed a lot better and told her the voices were quieter.

On the day he killed Ensink, he had arrived at his sister’s flat early in the morning, waking her up “because he was worried about me and that my life might be in danger”, she said. She reassured him, and went back to bed. Later she became aware of a stabbing in the street and told officers she thought the person they were looking for was her brother. He was arrested after police gave chase on foot.

Nandap admitted killing Ensink and was convicted of manslaughter due to diminished responsibility. He was sentenced to an indefinite hospital order at Broadmoor in October 2016.

The inquest, which is due to last three weeks, continues.

