Representative Elijah Cummings of Baltimore has died aged 68, his office said in a statement Thursday morning.

Cummings was the chairman of the House oversight committee, and was a key part of the Trump impeachment probe.

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Representative Elijah Cummings of Baltimore has died aged 68 after health complications in the hospital, his office said in a statement Thursday morning.

In the statement, his office said that he died at around 2.45 am at Johns Hopkins Hospital in Baltimore. His office did not specify the cause of his death, but according to reports Cummings had previously been treated for heart and knee issues.

As chairman of the House Oversight Committee, Cummings had played a key role in the impeachment inquiry into President Donald Trump.

Cummings had represented Maryland's 7th congressional district since 1996.

The Baltimore Sun reported that he hadn't returned to work after going to hospital in September for treatment that he had said should only keep him from work for about a week.

He missed two roll call votes last Thursday, the first day back after congressional recess, according to The Washington Post.

On Twitter, tributes were paid to the congressman as news of his death broke.

The veteran congressman had continued to play an active part in the impeachment inquiry into President Trump during his treatment, according to local politics site Maryland Matters.

It said he had been consulting with other Democratic leaders, had issued a subpoena, and was working on another.

He was one of three congressional leaders who in September signed a letter that accompanied a subpoena issued to Secretary of State Mike Pompeo, who has declined to comply with the inquiry.

He had been subjected to a barrage of criticsm in July by Trump, which House Speaker Nancy Pelosi and other critics denounced as racist.

The president described Cummings' Baltimore constituency as "a disgusting, rat and rodent infested mess" and "the worst run and most dangerous anywhere in the United States."

Cummings said of the president's rhetoric in an interview with NPR:"We must also stop the hateful, incendiary comments. We've got to do it. Those in highest levels of government must stop invoking fear, using racist language and encouraging reprehensible behavior."

In a tweet Thursday, Trump praised the congressman.

A Baltimore native, Cummings was a lawyer before his election to congress.