Rep. Elijah Cummings (D-Md.) died following complications concerning longstanding health challenges, his office announced Thursday morning. He was 68.

Cummings served as chairman of the House Oversight and Reform Committee and was a key leader in the ongoing impeachment inquiry into President Donald Trump. He had represented Maryland’s 7th Congressional District since 1996.

“My warmest condolences to the family and many friends of Congressman Elijah Cummings,” President Trump tweeted Thursday morning. “I got to see first hand the strength, passion and wisdom of this highly respected political leader. His work and voice on so many fronts will be very hard, if not impossible, to replace!”

Cummings died at Johns Hopkins Hospital at approximately 2:45 a.m., according to his office. He had been absent from Capitol Hill in recent weeks as he addressed his health and had not participated in a roll call vote since Sept. 11.

Rep. Carolyn Maloney (D-N.Y.) will serve as acting chair of the House Oversight and Reform Committee following Cummings’ death, as she was next in seniority. The search for a permanent chair will take place at a later time.

“Today, our nation has lost a true leader and pioneer, and I have lost a dear friend and mentor. You will be missed,” Maloney tweeted of Cummings.

Cummings is survived by his wife Maya Rockeymoore Cummings, who is chairwoman of the Maryland Democratic Party, and three children.

“Congressman Cummings was an honorable man who proudly served his district and the nation with dignity, integrity, compassion and humility,” Rockeymoore Cummings said in a statement. “He worked until his last breath because he believed our democracy was the highest and best expression of our collective humanity and that our nation’s diversity was our promise, not our problem. It’s been an honor to walk by his side on this incredible journey. I loved him deeply and will miss him dearly.”