Sarah Obama, the step-grandmother of Barack Obama, died on Monday at age 99 in Kenya while receiving treatment for an undisclosed ailment, according to authorities.

The former U.S. president took to Twitter shortly after the news broke to honor his grandmother, who helped raise Obama’s father as the third wife of his paternial grandfather, Hussein Onyango Obama.​​

“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,” Obama wrote. “We will miss her dearly, but we’ll celebrate with gratitude her long and remarkable life.”

Obama had previously called Mama Sarah “granny” in his book Dreams from My Father, and visited Kenya several times both in and out of office. In 2015, Obama became the first U.S. president to visit Kenya and met with Mama Sarah, who was previously a cook for British missionaries and later helped Kenyan children get an education through her foundation

“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,” said Kenyan President Uhuru Kenyatta in the statement.

The former president’s half-sister Auma also honored Mama Sarah on Twitter.

“Just lost the most important person in my life – my gran, Mama Sarah,” Auma wrote. “My heart is broken! But as I write, not able to stop the tears from pouring, I know I was blessed to have her for so long! My inspiration, my rock, my comfort zone, my safe space. Rest in peace Dani!”