Video report by ITV News Correspondent Sejal Karia
The widow of a lecturer stabbed to death as he delivered leaflets announcing his daughter's birth has asked why his psychotic killer was on the streets.
Dr Jeroen Ensink, 41, was killed in Islington, north London, by a mentally ill man 11 days after becoming a father on December 29 last year.
On Monday, Femi Nandap, 23, was detained in Broadmoor psychiatric hospital indefinitely after admitting the manslaughter of Dr Ensink - a lecturer at the London School of Hygiene and Tropical Medicine.
Nandap, who ITV News believes may be deported to Nigeria, stabbed Dr Ensink days after charges of knife possession and assaulting a police officer against him were dropped.
Dr Ensink's widow, Nadja Ensink-Teich, 37, has now told ITV News she wants to know why Nandap, held at Broadmoor high-security psychiatric hospital following the killing, was on the streets at the time.
Before the knife charges were dropped, Nandap had also breached his bail conditions by travelling to Nigeria.
At a previous court hearing, prosecutors said Nandap had suffered "an abnormality of mental function" shortly before the attack.
"My big question from the very beginning was 'how this could ever have happened?'", Mrs Ensink-Teich told ITV News.
"I still can't get my head around it. If someone is so mentally unfit, how could they be allowed to be at liberty to walk around with knives?".
She added: "This should never have happened. Somewhere along the line there was a failing".
Mrs Ensink-Teich described her husband, a renowned expert in sanitation and water management in countries including Ethiopia, Senegal, Vietnam, Afghanistan and Sri Lanka, as her "soul mate" and an "absolutely brilliant man".
His daughter, Fleur, was born just days before his death yards from their home.
Mrs Ensink-Teich said she had "every right" to be angry towards Nandap, but that would not help her to "live life by love " - the way she wishes to raise her daughter.
"The person I would be hurting most by having those [bitter] feelings is myself, our daughter, that of our friends and families", she said.
"I don't want to be hard, to be bitter. It's difficult enough the way it is".
Since Dr Ensink's death, his widow and the London School of Hygiene and Tropical Medicine have set up a memorial fund in his honour, raising more than £50,000 so far to help further his work.