Hong Kong  - A Hong Kong brother and sister allegedly killed by their mother died two hours after police had visited their home, an autopsy report revealed Friday.

The two, aged 13 and 12, were found dead Wednesday afternoon after their 38-year-old mother called police to tell them they were unconscious after she had drugged them with orange juice laced with sleeping pills and burned charcoal in the flat.

However, both fire officers and police had made an earlier visit to the flat 13 hours before after a neighbour reported smoke coming from the flat, but the officers had walked away, deciding there were no suspicious circumstances.

Now it has emerged from autopsy results that the children died from inhaling carbon monoxide, a product of burning charcoal, two hours after the first police visit.

The mother, Lee Fung-yee, is believed to have lit the charcoal after arguments with her married lover, the father of the children.

Police said that on their first visit, Lee denied them access to the flat in the Tuen Mun district, saying she was burning incense and did not want the children disturbed from sleep.

When fire officers broke into the flat 13 hours later after her telephone call, Lee was in a semi-conscious state, but the children were dead. She was recovering in hospital but has not been charged.

The incident is the latest in a series of family tragedies in the city of 7 million people.

There have been eight deaths involving five families so far this year with charities and support groups working with families in crisis blaming the widening gap between the rich and poor, the high-stress levels of families trying to cope with the economic downturn and the lack of support in the community. (dpa)