It has emerged that when he was arrested, Mr De Gelder's only words were: "I have a question".
The words echo a scene in Batman: the Dark Knight when the Joker violently gatecrashes a party looking for a the fictional attorney general of Gotham City, Harvey Dent.
"I only have one question: where is Harvey Dent? I'll settle for his loved ones," says the character played by Ledger, before threatening a woman with a knife.
Patrick De Mey, the police officer who arrested Mr De Gelder, described the suspect as being in a passive and trance-like state."
"When I asked him to step off his bicycle and put his backpack on the ground he obeyed perfectly," he told the Gazet Van Antwerpen.
"But in answer to my questions, he clearly had no understanding. He continued to stare at me all the time with wide open eyes and a large grin on his face."
Since his capture, he has not spoken and has gone on hunger strike.
He was reported to be on an intravenous drip in a medical facility in a prison in Bruges on Sunday night.
Ledger died of a drug overdose on Jan 22, 2008, a year and a day before Mr De Gelder is accused of carrying out the attack.
According to witness statements and police drawings, the suspect had painted his face white, blackened his eyes and had coloured his hair red.
His gruesome and bizarre garb prompted investigators to make the Joker comparison and make a link to another cult Goth film, The Crow, which inspired Ledger in his Batman role.
Belgium's Le Soir newspaper has even speculated on the significance of the fact that the name Gelder is an anagram for Ledger.
Mr De Gelder will be examined by three psychiatrists before a closed court hearing on Tuesday decides if he is mentally responsible for his actions.
The young man, who lived his whole life within three miles of the family home, in the Eksaarde district of Sinaai, has emerged as a troubled loner.
Former classmates at the Roman Catholic Sint Carolus school, in the town of Sint Niklaas, described him as a fan of Goth music who could sometimes behave oddly.
He was nicknamed Satan because of his long black hair."
A 20-year-old man has been charged with killing two toddlers and a carer in a frenzied attack at a nursery.
Ten other children, including a one-month-old baby, were knifed by the man who had smeared black and white warpaint on his face and sprayed his hair red.
Two further adults were seriously injured after the rampage in Dendermonde, Belgium, one suffering face and stomach wounds.
Police are still trying to establish a motive for the attack at the Fabeltjesland creche - which means in English, 'the land of fables'.
The attacker apparently tricked his way into the nursery yesterday morning, then ran amok stabbing children and staff before escaping by bicycle. The dead carer has been named as Marita Blindeman.
Prosecutor Christian Du Four said: 'He did not escape from a psychiatric institution. He was not under the influence of alcohol or drugs,' he said, referring to reported rumours about the suspect.
Mr Du Four said the weapon had been found, describing it as a 'large knife' about 12in long.
The deaths left Dendermonde, half an hour north west of Brussels, in shock.
Members of the Belgian royal family have gone to the town to visit the scene and speak to emergency workers.
About 18 children, some just babes in arms, were at the creche when, police say, a man knocked on the door and was let in.
He immediately produced knives and lashed out at the children - including those being held by staff at the day-care centre.
The attacker is believed to come from nearby Grembergen and to harbour a grudge against children. He had no connection to the nursery, according to officials.
At the bloodstained nursery, medics were in tears as they treated the child victims, who were all under six. Some rescuers had children at the centre.
Only nine of the 21 children in the building escaped unhurt.
Police showed digital photographs of the injured children to parents and relatives in an attempt to identify them. They put numbers on the heads of each child victim to help the process.
'There was blood everywhere and of course there was panic,' said Theo Janssens, a senior member of Dendermonde's department of social services who was on the scene within five minutes.
'It was horrible. You can't imagine it. The two who were killed were just babies. One was already dead when I got there and the other was in a very bad way.
'One of the adult staff was also already dead.'
Parents comfort each other at a crisis centre after one adult and two children were stabbed to death at the nursery in Dendermonde
Police said the attacker, who was unknown at the creche, muttered: 'I have a question,' as he was let in the front door and then began lashing out with a knife.
Dendermonde's chief of police, Paul Puttemans, said the attack began when a man arrived at the creche on a bicycle - wearing a bullet-proof vest.
Mr Puttemans said he was believed to be carrying several knives and possibly other weapons.
After carrying out the frenzied attack in Dendermonde near Brussels, the knifeman
made his escape on a bicycle. He was arrested shortly afterwards at a clinic where he had been receiving in-patient treatment. [Edit: info from Daily Mail (I checked again and this has not been removed from the story), I doubt they just made it up; if this isn't true then apologiez, but if this was the case in reality then I'm sure the "official" stories would be saying it isn't true as part of the cover-up (changed size anyway)]
He fought back against the policemen who detained him, injuring himself and two officers.
Surgeon Ignace Demeyer, who is hospital co-ordinator for emergencies in the Dendermonde region, said he hoped that the children caught up in the attack were young enough not to be scarred in later life by the memories.
Extra supplies of blood for transfusions were sent from Brussels to the local hospital where at least three of the injured children were said to be in a critical condition.
'But they will be physically scarred,' he said. 'We have 10 children, what I would call toddlers, in hospital being treated for wounds. They will all survive, as well as two adults working at the centre.'
Asked how one man had been able to attack so many in just a few moments with a knife, Mr Demeyer replied: 'The truth is that stabbing children is very easy, especially when they are very small, and some of these would have been sleeping.'
A 20-year-old man has been charged with the killings at the daycare centre
'For those who survived, this was horribly traumatic and they are already being put into post-traumatic stress programmes.'
Piet Buys, mayor of Dendermonde, said: ‘The killing zone has been cordoned off and the victims are being attended to.
‘Those who were not injured have been brought to a central crisis centre for care and counselling.
'They are in deep shock and need help. We are devastated by this. We do not know who the perpetrator is or his motives.’
Belgium’s interior minister, Zaken Guido De Padt, promised a review of security in kindergartens across the country.
'We must ask how this assailant was able to gain access to the premises to commit this terrible crime,' he said.
The man was known to local police and had been arrested on more than one occasion for drug-related offences.He was taken to a police station in Brussels usually reserved for terrorist suspects.