The mother of two babies found dead at their home was arrested on suspicion of murder yesterday.
The children's father, unofficially named as Jeff Boots, 34, tried to revive his 10-week-old son and one-year-old daughter when he arrived home from work on Wednesday night.
The children were pronounced dead by paramedics called to the £1.2m house in Wandsworth, south-west London.
The mother, named as Felicia Boots, also 34, is said to have cut her wrists but did not require hospital treatment.
Detectives are understood to be investigating the possibility that she was suffering from post-natal depression. Neither of the children had any visible injuries and police believe they may have been smothered. Post-mortem examinations were due to take place to establish the cause of their deaths.
Officers from the Metropolitan Police's child abuse command, which is leading the investigation, said they were not looking for anyone else in connection with the deaths. Forensic experts were seen going in and out of the house, which remained cordoned off last night.
The London Borough of Wandsworth's social services department said the children were not known to the authority and were not on an at-risk register. Wandsworth Primary Care Trust did not respond to calls.
A neighbour, Jackie Cook, 52, said the family had moved into the semi-detached property only a couple of weeks ago from a penthouse apartment nearby. The affluent suburb of Wandsworth is popular with middle-class professionals. The couple are thought to have moved to the UK from Canada.
Ms Cook said: "It's such a tragedy, I can't believe it... There were police swarming everywhere. There were ambulances and police cars all along the road." Another neighbour, a 27-year-old man, spoke of the community's shock: "They had only just moved in so we hadn't had the opportunity to introduce ourselves. We were going to go over and say hello this weekend.
"Forensic teams were still going in and out of the house in the early hours of [yesterday] morning... It's a lovely community with a great spirit and no one can believe what has happened. It's a terrible tragedy."
At least one new mother in 10 goes through post-natal depression, which can range from a relatively mild to a life-threatening condition. A small number of women with very severe depression develop psychotic symptoms. They may hear voices and have unusual beliefs.
Depressed mothers often worry that they may harm their babies, but this is very rare. According to the charity Mind, some evidence suggests that around half of these women are afraid to tell health visitors in case social services take away their children. A statement from the Metropolitan Police confirmed the investigation yesterday, saying that they were called to the house in Killarney Road at 7.25pm on Wednesday. They were not able to say when the post mortems will be complete but added that next of kin had been informed.