Sarah Obama, former president's step-grandmother, dead in Kenya hospital

Former President Barack Obama's step-grandmother died at around 4 a.m. local time in Kenya's third largest city, Kisumu, on Monday.

Known as the family's matriarch, Sarah Obama was at least 99 years old and promoted education for girls and orphans in her rural Kogelo village.

Relatives and officials confirmed her death on Monday. According to a family spokesman, the former president had been informed of the death and has sent his condolences.

The former president also tweeted a tribute, saying his family was mourning her loss. "My family and I are mourning the loss of our beloved grandmother, Sarah Ogwel Onyango Obama, affectionately known to many as 'Mama Sarah' but known to us as 'Dani' or Granny," he said.

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"We will miss her dearly, but we’ll celebrate with gratitude her long and remarkable life."

In a separate statement, he reflected on her influence on his family's life. 

"Although not his birth mother, Granny would raise my father as her own, and it was in part thanks to her love and encouragement that he was able to defy the odds and do well enough in school to get a scholarship to attend an American university," he said.

"When our family had difficulties, her homestead was a refuge for her children and grandchildren, and her presence was a constant, stabilizing force. When I first traveled to Kenya to learn more about my heritage and father, who had passed away by then, it was Granny who served as a bridge to the past, and it was her stories that helped fill a void in my heart."

"During the course of her life, Granny would witness epochal changes taking place around the globe: world war, liberation movements, moon landings, and the advent of the computer age. She would live to fly on jets, receive visitors from around the world, and see one of her grandsons get elected to the United States presidency. And yet her essential spirit - strong, proud, hard-working, unimpressed with conventional marks of status and full of common sense and good humor - never changed."

Her foundation, The Mama Sara Obama Foundation helped provide food and education to children who lost their parents — providing school supplies, uniforms, basic medical needs, and school fees. The United Nations honored her in 2014 with the inaugural Women’s Entrepreneurship Day Education Pioneer Award.

Sarah Obama, affectionately known as Mama Sarah, was being treated at the Jaramogi Oginga Odinga Teaching and Referral hospital in Kisumu, according to her daughter Marsat Onyango. 

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The matriarch is expected to be buried Tuesday before midday and the funeral will be held under Islamic rites.

"The passing away of Mama Sarah is a big blow to our nation. We've lost a strong, virtuous woman, a matriarch who held together the Obama family and was an icon of family values," Kenyan President Uhuru Kenyatta said.

The Associated Press and Fox News' Lillian LeCroy contributed to this report.