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(CNN) —  

Kay Hagan, a former US senator from North Carolina and the first female Democratic senator to represent the state, died Monday, her family said in a statement. She was 66.

“We are heartbroken to share that Kay left us unexpectedly this morning,” the Hagan family said. “Kay meant everything to us, and we were honored to share her with the people of North Carolina whom she cared for and fought for so passionately as an elected official.”

Hagan, a Democrat, served in the Senate from 2009 to 2015 after defeating Elizabeth Dole. She lost her reelection bid in November 2014 to Republican Sen. Thom Tillis. Tillis said in a tweet Monday that he was “heartbroken” by Hagan’s passing and remembered her for her “dedicated and distinguished record of public service to our state & nation.”

Hagan is survived by her husband, Chip Hagan, and children Jeanette Hagan, Tilden Hagan and Carrie Hagan Stewart. They remembered her “humor and spirit as the hub of our family” and said in their statement that she had a knack for making people feel welcome.

The family did not provide information on the cause of her death, but said they “are deeply grateful for the support shared with our family as Kay worked to regain her strength these last few years after her illness.”

The News & Record, a North Carolina newspaper, reports Hagan was in Washington in 2016 when she fell ill and was admitted to a hospital with encephalitis, an inflammation of the brain. The illness led to a months-long hospitalization at an Atlanta rehabilitation hospital, according to the paper, and her family later said the illness was caused by Powassan virus, which is transmitted to people from ticks.

In June, Hagan made a rare public appearance at a groundbreaking ceremony for an air traffic control tower in the state – a project she was credited with advancing during her time in Congress, according to the News & Record. Her husband told the paper at the time that her illness limited her speech capabilities and caused standing and walking difficulties. Though she did not deliver public remarks, she “shared private greetings with well-wishers” at the event, the paper said.

Former Vice President Joe Biden said in a statement Monday that he had visited with Hagan in Durham, North Carolina, on Sunday, and remembered her as “a courageous soul who lived every day of her too-short life with incredible dignity and character, even as the days became more difficult physically.”

“She was a champion for North Carolina and a fierce defender of all its citizens. She stood for women’s rights and marriage equality, not because it was politically popular, but because it was right,” Biden said. “As a United States Senator, she was a crucial partner to our administration to pass both the Recovery Act and the Affordable Care Act.”

Hagan’s “legacy of service will continue to live on in new leaders she inspired to follow in her steps,” he said.

Former President Barack Obama also offered his condolences to Hagan’s family and remembered her as a “terrific public servant” who was “eager to find common ground, willing to rise above the partisan fray, and always focused on making progress for the people she served.”

“As President, I deeply appreciated her reasoned, pragmatic voice, whether we were working together to pass the Affordable Care Act, reform Wall Street, support working families, or just make Americans’ lives a little better,” he said in a statement on Monday. “Her record is one all public servants would do well to follow, and her perspective is one we’ll sorely miss.”

Sen. Richard Burr, a North Carolina Republican who served alongside Hagan, said in a statement that he and his wife “are deeply saddened by the sudden and untimely loss” of his former colleague.

“In our time as Senate colleagues, we worked across the aisle together frequently on issues that we both knew would determine what type of country our children would inherit, from conservation to our common defense,” Burr said, noting that Hagan will be remembered “for her tireless work on behalf of the home and the people she loved.”

This story has been updated.

CNN’s Sarah Mucha, Jeff Zeleny and Chandelis Duster contributed to this report.