The attack was the largest mass killing in the city in at least three decades.
The police said they believed the victims were killed with “an edged weapon” but would not provide any more details.
He was arrested shortly after police received two emergency calls around 11 pm on March 6 to the home in a residential neighbourhood, police said. The victims were identified as Ms Darshani Banbaranayake Gama Walwwe Darshani Dilanthika Ekanyake, 35, the mother of the four children killed; Inuka Wickramasinghe, a seven-year-old boy; Ashwini Wickramasinghe, a four-year-old girl; Rinyana Wickramasinghe, a two-year-old girl; and Kelly Wickramasinghe, a two-month-old girl.
The sixth person killed, Mr Amarakoonmubiayansela Ge Gamini Amarakoon, 40, was an acquaintance who had also arrived from Sri Lanka and was living with the family. Ms Shanti Ramesh, a neighbour, said she saw about 15 police cruisers rush to the home the night of the attack.