MSNBC's Lawrence O'Donnell explained Wednesday night how a new study is completely undercutting President Donald Trump's claims that the government responded well to Hurricane Maria last year when the storm devastated Puerto Rico.

"President Trump never talks about the number of people killed in the hurricanes in Puerto Rico last year, but he does give himself an A+ for the government's response," ODonnell said on "The Last Word."

He continued: "The official death count in Puerto Rico is 64, but a new Harvard study, published in the New England Journal of Medicine says that Hurricane Maria in Puerto Rico was the deadliest storm in American history. 4,645 people actually lost their lives because of that storm."

Many people died, the study found, because of delayed or prevented medical care as a result of the storm, and many died as a direct result of the hurricane's impact on the island. Advocates for Puerto Rican survivors of the storm have long argued that the official death count was a wild underestimation of the storm's toll, the effects of which are still being felt across the island territory.

If the new figures are accurate, the storm was more than twice as deadly as Hurricane Katrina in 2005, and it took more lives than even the attacks on 9/11.

Watch the clip below: