Louis Tomlinson's sister Felicite Tomlinson was found dead at her home in London this week.
Entertainment Tonight reported Thursday Felicite died at age 18 on Wednesday after a suspected heart attack.
Sources said Tomlinson was notified of his sister's death Wednesday night. The singer and his family are said to be devastated and Tomlinson has canceled his upcoming appearances to mourn.
"At this stage the death is being treated as unexplained. A post-mortem examination will take place in due course. Enquiries continue," the Metropolitan Police Service said.
TMZ said Felicite collapsed from a heart attack at her studio apartment Wednesday in London. Someone at the apartment called an ambulance, but Felicite was pronounced dead at the scene after paramedics arrived.
Sources said Felicite had no warning signs or known prior history of heart trouble. The teenager was an aspiring fashion designer and had more than 1 million followers on Instagram.
Felicite was one of Tomlinson's six siblings. The Late Late Show host James Corden was among those to voice their sympathies for the singer online.
"Such incredibly sad news today. You're not on your own in this @Louis_Tomlinson So many people are pulling for you and your family right now," he tweeted.
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Felicite's death comes just over a week after Tomlinson released a new song honoring their late mom, Johannah Deakin. "Two of Us" debuted March 7, more than two years after Deakin died of leukemia.
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