Former North Carolina Sen. Kay Hagan died Monday at the age of 66, NBC News has confirmed.

Hagan, a Democrat, served one term in the Senate from 2009 to 2015 after defeating Republican Sen. Elizabeth Dole in 2008. She lost her seat in her first re-election race, in 2014, to Thom Tillis. Prior to her time in Congress, she served a decade in the North Carolina state Senate.

In a statement to NBC News, Hagan’s family said Hagan had died after a three-year battle with encephalitis, a brain inflammation that can be fatal.

"We are heartbroken to share that Kay left us unexpectedly this morning," the Hagan family said in a statement. "We are deeply grateful for the support shared with our family as Kay worked to regain her strength these last few years after her illness, and we appreciate your continued prayers."

Hagan had been diagnosed in 2016 with Powassan virus. That virus is known to cause encephalitis and is transmitted to humans by ticks. There is no cure for the virus.

According to the Associated Press, Hagan spent time in January 2017 at an Atlanta rehabilitation center after being diagnosed with the virus.

Hagan is survived by her husband Chip Hagan and their three children.

Lawmakers at all levels paid tribute to Hagan after news of her death emerged.

"Michelle and I join all those in North Carolina and across the country in mourning the loss of Senator Kay Hagan," former President Barack Obama said in a statement. "She was, quite simply, a terrific public servant— eager to find common ground, willing to rise above the partisan fray, and always focused on making progress for the people she served.

Former Vice President Joe Biden, a 2020 presidential candidate, said in a statement that he had seen Hagan on Sunday during a trip to North Carolina and called her 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."

Democrats in North Carolina also offered condolences.

"Kristin and I are deeply saddened to learn of the passing of our dear friend Senator Kay Hagan," North Carolina Gov. Roy Cooper, a Democrat, said in a statement. "Kay was a fierce advocate for North Carolina, and she represented our state with courage and grace her entire career."

"North Carolina is mourning one of our best today," Cooper added.

Rep. Mark Walker, D-N.C., said, "My heart goes out to Chip Hagan and the entire Hagan family in the passing of Kay."

"Sen. Hagan was dedicated to serving North Carolinians," Walker added in his statement.

The world is a little grayer since we lost this amazing woman and dear friend today. The courage and determination that Kay Hagan and her family showed in the face of her debilitating illness over the last few years was awe inspiring. I loved her. pic.twitter.com/nzvuN8eALd — Claire McCaskill (@clairecmc) October 28, 2019

Some of Hagan’s former U.S. Senate colleagues also spoke out.

Sen. Richard Burr, R-N.C., who served alongside Hagan during her time in the Senate, said he was "deeply saddened by the sudden and untimely loss of Kay Hagan."

"Kay dedicated much of her life to serving North Carolina, and she will be remembered for her tireless work on behalf of the home and the people she loved," Burr said in a statement.

Former Sen. Claire McCaskill, D-Mo., tweeted that "the world is a little grayer since we lost this amazing woman and dear friend."