Officers forced their way into the house on Staines Road, Bedfont a short time later and found the bodies of four people inside.
Police said they believe they know the identity of the four people, thought to be a man and woman in their 30s, an 11-year-old girl and a three year-old boy. The force confirmed they are all from the same family.
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A local resident has described the family as “normal neighbours”, while another has said the family “kept themselves to themselves”.
Enquiries are underway to formally identify the four people and inform their next of kin. The Met’s investigation is led by homicide detectives from Specialist Crime. The force said it is not currently looking for anybody else in connection with the incident.
Forensic investigators have arrived at the house in Hounslow. They could be see gathered around the entrance to the building, with some wearing face masks and gloves.
A man who lives next door, who did not wish to be named, told reporters the family “were just normal neighbours”, adding: “We just had greetings here and there, that kind of stuff.”
He described coming home from work and seeing “police everywhere”. “It was really busy,” he said. “Nothing like that happens here. I have no idea what happened.”
Michelle Cunningham, 49, who lives nearby and laid flowers at the scene, said: “It’s so sad.
“They were an eastern European family, kept themselves to themselves. I used to see the man outside his house sometimes.”
Seema Malhotra, MP for Feltham & Heston, has said that “everyone will be devastated by the news”.
She wrote on social media: “Everyone will be devastated by the news that two adults and two children have been found dead in their home in Bedfont.
“Specialist investigations are ongoing. Our thoughts are with their family.
“If you have any information please do get in touch with the police.”
Chief Superintendent Sean Wilson, local policing commander in west London, said: “Our investigation is at a very early stage, and based on our initial enquiries, we are not currently seeking anybody else in connection with the incident.
“I know the shock and distress that this terrible incident will cause among the community in Hounslow and beyond. I can assure local people that specialist officers are working to establish exactly what happened and I will provide further information as soon as I can.
“I am also aware that this incident will be particularly upsetting for children, and I ask people to please be responsible about what they post on social media, including not naming or speculating as to the names of those who’ve died.
“There will be additional police officers in the area across the weekend, I urge anyone with information or concerns to speak with those officers.”