U.S. Sen. Frank Lautenberg (D-N.J.) died Monday morning due to complications from viral pneumonia, his office said in a statement Monday morning.

The Bergen Record first reported the news.

At 89, Lautenberg was the oldest member of the Senate. He was also the last veteran of World War II serving in the Senate.

He had been battling a number of health issues this year, missing several votes while spending time outside of Washington. In April, he memorably returned to the floor to vote for an amendment that would have expanded background checks on gun purchases.

Lautenberg's death opens up a seat in the U.S. Senate that will be filled by New Jersey Gov. Chris Christie, a Republican.

Lautenberg served three terms in the Senate before originally retiring after the 2000 election. He returned to the Senate in 2003. In the 2002 election, he became a late replacement on the ballot for Sen. Robert Torricelli (D-N.J.), who withdrew while facing corruption charges.

He had already announced that he did not plan to seek re-election in 2014.

His office sent out a list of Lautenberg's accomplishments during his term: