Two men died and five people, including a child and an infant, were taken to hospital following a suspected carbon monoxide leak.
The pair, thought to be aged 38 and 42, were found at a multiple occupancy house in north London on Sunday afternoon, Scotland Yard said.
Two other men, a woman and two boys were also found at the property in Bacon Lane, Edgware.
Neighbours spoke of their shock at the deaths, which are believed to have been caused by a faulty boiler.
The occupants of the rented multi-occupancy semi-detached property are thought to have moved in just weeks ago.
One neighbour, who thought they were from Eastern Europe, said: “It’s really shocking, very sad, the loss of life. It doesn’t matter how it happens, but it’s really sad.”
The man, who works in the medical profession, said: “It’s a rented property. I know the owner, I’ve met him.
“He was renovating just after Christmas. I don’t know what they did inside, but I think they did carpets, painting.”
The windows of the house were left open on Sunday evening, while a gas engineer arrived to reconnect the gas to neighbouring houses.
Another neighbour said the residents had only been living in the house for three to four weeks.
“They are fairly new. As far as I know they’re very quiet,” he said.
“They have only been here a short period of time, they have been very quiet.
“I have never spoken to them, they kept themselves to themselves.”
The neighbour, who did not want to be named, said there had been problems with previous tenants.
“There was quite a few of them and they were very noisy, always having their barbecues in the back,” he said.
“He’s (the landlord’s) always had problems with multiple occupancy, but these last lot have been the best so far.”
He also said there was some renovation work carried out before the new tenants moved in.
He added: “I’m shocked, really saddened because they seemed quite nice.
“My wife was saying she wishes she introduced herself sooner.”
Scotland Yard said the deaths were being treated as “unexplained”.
Officers discovered the two dead men when they arrived at the scene around 1.30pm, along with fire crews and paramedics.
“At this early stage, police believe that there was a possible carbon monoxide leak at the address,” the force said in a statement.
“Police in Harrow continue to deal alongside the Health and Safety Executive who have been informed.”
A London Fire Brigade spokeswoman said: “We have had two males pronounced life extinct by London Ambulance Service.
“Two adult males, one adult female, one male child and one male infant have also been treated on site for carbon monoxide poisoning and taken to hospital by London Ambulance Service.”