Austrian dungeon captive is reunited with mother.
Interviewed on television outside their home in the city of Aomori, in northern Japan, the father of Tomohiro Kato said: "We are terribly sorry for the people who lost their lives and were injured by what our son has done."
"No matter how many times we apologise, we cannot apologise enough," said Kato, 49, a bank worker.
Neither of the parents' first names have been released and their faces were obscured to protect their privacy.
Seven people died and 10 others have been treated at hospitals for knife wounds after Kato drove a hired lorry into a crowd of pedestrians before leaping from the cab and indiscriminately stabbing passers by.
As her husband spoke, Kato's 53-year-old mother wiped away tears before collapsing on the ground.
"I still can't say what I could do (for the victims) as I'm very confused right now," Mr Kato said.
Police on Wednesday revealed that Kato was carrying five knives when he began his rampage, including the survival knife that he used on most of the victims.
There have also been more indications of his state of mind and what might have triggered the attack, with hundreds of messages to Internet boards revealing loneliness, resentment and anger at his inability to make friends or find a girlfriend.
Kato was also apparently resentful of his parents, writing, "I have been forced to play the good boy since I was little."
He complained that his parents had sometimes done his homework for him as they wanted him to "look perfect," Japanese media reported.
"As they wanted to boast about me to other people, they would finish everything to make me look perfect," he wrote.
Prosecutors are expected to seek the death penalty.