Rep. Elijah Cummings, a longtime Maryland Democrat and key figure leading investigations into President Donald Trump, has died at age 68, his office announced early Thursday morning.

He died of "complications concerning longstanding health challenges," his office said in a statement.

The congressman, who had represented Maryland's 7th Congressional District since 1996, served as the chairman of the House Oversight and Reform Committee, one of the panels involved in the impeachment inquiry of Trump.

He oversaw a range of investigations into the Trump administration, from issues relating to the impeachment inquiry to the treatment of migrants at the southern border to the use of personal email for official use by White House officials to how a citizenship question was considered for the US census.

And it was his committee that grilled Trump's former personal attorney, Michael Cohen, in a blockbuster hearing this past February.

It was not immediately clear who will succeed Cummings as chairman of the Oversight Committee or how his passing will affect the swirling impeachment investigation into Trump.

Cummings had been in failing health in recent weeks. He had been in and out of the hospital, missing votes and business in his committee. He was spotted several times with a breathing tube in his nose connected to oxygen while sitting on the House floor. When speaking to reporters, he would have to wait for 15 to 20 seconds or so to catch his breath before speaking. He would drive around on a motorized wheelchair through the Capitol and then walk in using a walker.

Although he was chairman of the Oversight Committee, he had not been in command of the investigations on his panel. His staff did a lion's share of the work and his staff has been helping lead the charge in the impeachment inquiry.

Prominent Trump critic

As he has led the investigative efforts, Cummings also clashed publicly with the President. Over the summer, Trump tweeted disparaging remarks toward Cummings and his Maryland district, which includes much of Baltimore, calling the majority black district a "disgusting, rat and rodent infested mess."

Responding to some of the President's tweets -- in which Trump suggested the congressman needed to spend more time fixing his district -- Cummings said on Twitter: "Mr. President, I go home to my district daily. Each morning, I wake up, and I go and fight for my neighbors. It is my constitutional duty to conduct oversight of the Executive Branch. But, it is my moral duty to fight for my constituents."

Despite that high-profile feud, Trump and Cummings did not always disagree. More than two years prior, Cummings emerged from a White House meeting with Trump and told reporters that the two men had found common ground on their shared interest in lowering drug prices.

At the time, Cummings also said he urged the President to rethink his language on African American communities after Trump repeatedly painted a grim picture of inner-city life on the campaign trail.

"I want you to realize that all African American communities are not places of depression and where people are being harmed," Cummings told reporters, recalling his conversation with Trump. "When we hear those words about carnage and we are living in depressed situations, I told him it was very hurtful."

In another high-profile moment earlier this year, Cummings stood up for Republican Rep. Mark Meadows, one of the President's closest allies and staunchest defenders in Congress, in the face of accusations of racism. The chairman referred to Meadows as one of his "best friends." When Meadows learned of Cummings' passing Thursday, he said he was "truly heartbroken."

"I have no other words to express the loss," he told CNN's Dana Bash.

Leading African American voice on Capitol Hill

Cummings, who grew up during the Civil Rights Movement, had become a leading voice among African American lawmakers on Capitol Hill at the time of his passing, and his death triggered an outpouring of grief from his colleagues.

"He spoke truth to power, defended the disenfranchised and represented West Baltimore with strength and dignity," Rep. Hakeem Jeffries, House Democratic caucus chairman and a fellow member of the Congressional Black Caucus, tweeted Thursday morning. "Congress has lost a Champion. Heaven has gained an Angel of Justice. May he forever #RestInPower."

House Judiciary Chairman Jerry Nadler called Cummings a "giant of public service," and Sen. Ben Cardin said his fellow Marylander "guaranteed a voice to so many who would otherwise not have one."

Rep. Debbie Wasserman Schultz told CNN's John Berman on "New Day" that Cummings was "a mentor and someone who in every situation would do the right thing, would put his community and the cause above everything else. Including himself."

The Florida Democrat said watching Cummings continue with his duties despite his health struggles was inspirational.

"We'll walk in his shadow, in his shoes that will never be filled," she said.

Veteran of Civil Rights Movement

CNN analyst: Cummings wanted to work for the people 01:17

Earlier this year, Cummings discussed how, even at a young age, he faced racial violence in trying to integrate parts of his neighborhood.

"We were trying to integrate an Olympic-size pool near my house, and we had been constrained to a wading pool in the black community," Cummings told ABC's "This Week" in July. "As we tried to march to that pool over six days, I was beaten, all kinds of rocks and bottles thrown at me."

Cummings said Trump's racist remarks against four minority members of Congress echoed the same insults he heard as a 12-year-old boy in 1962, which he said were "very painful."

"The interesting thing is that I heard the same chants. 'Go home. You don't belong here,' " he told ABC News' George Stephanopoulos. "And they called us the N-word over and over again."

Cummings was born and raised in Baltimore -- the city that is home to his district. The son of former sharecroppers, Cummings was born in 1951 and graduated from Baltimore City College High School in 1969.

He practiced law and served for 14 years in the Maryland House of Delegates, where, according to his congressional website, he became the first African American in Maryland history to be named Speaker Pro Tem.

In 1996, he was first elected to the US Congress. Cummings was reelected last year in the 7th Congressional District with 76% of the vote.

This story has been updated with additional developments Thursday. CNN's Kate Grise, Ashley Killough, Michael Warren, Daniel Allman, Matthew Hilk, Devan Cole and Veronica Stracqualursi contributed to this report.