The fire in a residential building in Philadelphia, Pennsylvania, in which at least twelve members of a family died on Wednesday, may have been caused by a child being ignited.

According to the search warrant, the ATF Federal Police Department has now started investigations into a five-year-old who is said to have played with a lighter near a Christmas tree.

In the fire, the deadliest in Philadelphia for more than 100 years, three sisters and at least nine of their children were killed early Wednesday morning (local time).

American media reported that the smoke alarms in the three-story row house in the Fairmount district were defective.

According to initial reports, almost 30 people lived in the old building, owned by the Philadelphia Housing Authority (PHA), which provides housing for socially disadvantaged Americans.

The PHA boss Kelvin Jeremiah has meanwhile rejected the allegation of possible overcrowding.

Ten years ago six people moved into the upper of the two apartments in the row house.

Since then, the family has had eight more children.

"We don't put people on the street just because they have children," said Jeremiah on Thursday.