Abu Dhabi has been under the de-facto rule of Crown Prince Mohammed bin Zayed since 2014, following Khalifa bin Zayed’s stroke, and who remained largely out of the public eye since.

Sheikh Khalifa bin Zayed Al Nahyan, the president of the United Arab Emirates, has died on Friday at the age of 73, according to state news agencies.

“The Ministry of Presidential Affairs announced that the UAE will observe a forty-day state mourning with the flag flown at half-mast starting today, and suspend work at all ministries, departments, and federal, local and private entities for three days,” the Emirates News Agency (WAM) wrote on Twitter on Friday.

Amir Tamim bin Hamad Al Thani offered his condolences to the family of deceased monarch and the United Arab Emirates’ nation in a tweet, saying: “With great sadness, we received the news of the death of my brother, His Highness Sheikh Khalifa bin Zayed Al Nahyan. As I express to his honorable family and the brotherly people of the Emirates, my deepest condolences and sympathy, I pray to the Almighty God to accept him with his mercy and forgiveness and dwell in his vast gardens. We belong to Allah and to Him we shall return.”

Born in 1948, Sheikh Khalifa bin Zayed was elected to succeed is father, the late Sheikh Zayed bin Sultan Al Nahyan, who served as the UAE’s first president from 1971 until his passing on November 2, 2004. Sheikh Khalifa was his eldest son.

He was the second President of the United Arab Emirates and the 16th Ruler of the Emirate of Abu Dhabi.

There has been no official statement on who will succeed him, yet.

According to Reuters, vice-president and premier Sheikh Mohammed bin Rashid Al Maktoum, the ruler of Dubai, will serve as president until the federal council, which brings together the rulers of the seven emirates, meets within 30 days to elect a new president.

Sheikh Khalifa has presided over a substantial transformation of both the federal government and the government of Abu Dhabi, since becoming the president of the UAE.

However, he has long ceased to be involved in day-to-day matters, with his brother, Abu Dhabi’s Crown Prince Mohammed bin Zayed, serving as the de-factor ruler.

He has been rarely seen in official images or at public events in recent years. He had a stroke a decade after his succession, and has remained largely out of the public eye since.

The world’s tallest tower, located in Dubai, United Arab Emirates, is named after the late monarch.