A young man who killed two babies and a nursing attendant in a stabbing rampage at a Belgian daycare has been linked to the slashing death of a grandmother.

Kim De Gelder, 20, is charged with killing a six-month-old boy, a nine-month-old girl and a 54-year-old caregiver last Friday at Fairyland nursery in the town of Dendermonde.

His lawyer Jaak Haentjens said De Gelder admits to the nursery killings, but denies involvement in the death of a 73-year-old grandmother on a remote farm the previous week.

De Gelder's motive for the nursery attack is unknown, Haentjens said.

"It remains a mystery," Haentjens said before his client appeared early Tuesday in front of a judge in Dendermonde. "I want to know why he did it. I am looking for the truth, whatever the truth may be."

"He obviously understands that he did something which is inhuman," said Haentjens, but he acknowledged that it was not clear whether his client felt any remorse.

Authorities said Gelder is a suspect in the elderly woman's death but has not officially been charged.

Investigators said they found blueprints in De Gelder's backpack of the nearby Sunshine and Toddler's Land nurseries, as well as an axe and knife.

De Gelder was placed on an energy-sustaining drip after refusing to speak or eat for three days, but began talking a bit on Monday.

He faces three murder charges and 12 attempted murder charges for the 10 children and two workers badly injured in Friday's attack.

When De Gelder entered the Fairyland nursery on Friday, witnesses said he only stated "I have a small question for you" before he began slashing his victims.

Then he calmly walked away and rode off on a bicycle before authorities arrested him outside Dendermonde, about 30 kilometres west of Brussels.

Belgian newspapers are referring to the young man as the "Joker" killer because he reportedly had painted his face white with black around the eyes. The Joker is the deranged villain in the latest Batman movie The Dark Knight and was played by late actor Heath Ledger.

Investigators said they found mascara in his backpack but it's not clear whether he intended to look like the Joker.