"The National Rifle Association of America is made up of four million moms and dads, sons and daughters - and we were shocked, saddened and heartbroken by the news of the horrific and senseless murders in Newtown," the NRA statement said.
"Out of respect for the families, and as a matter of common decency, we have given time for mourning, prayer and a full investigation of the facts before commenting."
Among members of Congress, Thompson, the former Sportsmen's Caucus co-chairman, was named to lead a Democratic task force on gun violence. He's a hunter, a wounded Vietnam veteran and a conservative Democrat.
"The only experience I've had with assault weapons was the one that I was trained with when I was in the Army," he said. "I know that this is not a war on guns. Gun owners and hunters across this country have every right to own legitimate guns for legitimate purposes and ... we are not going to take law-abiding citizens 'guns away from them."
On Monday, NRA member Manchin, Senate Majority Leader Harry Reid and Sen. Charles Grassley, R-Iowa — senior Republican on the Judiciary Committee — said it's time for a debate that would include gun control. Reid previously had taken pro-gun positions for years.
Not all Republicans were willing to go as far as Grassley or Kingston, but they didn't rule out tackling gun control.
Senate Republican leader Mitch McConnell told reporters, "The entire Congress is united in condemning the violence in Newtown and on the need to enforce our laws. As we continue to learn the facts, Congress will examine whether there is an appropriate and constitutional response that would better protect our citizens."
McConnell added that Reid controls the Senate schedule.
At a regular House Republican closed-door meeting Tuesday Rep. Tim Murphy of Pennsylvania, a psychologist, led a discussion on mental illness — which he described as the primary cause of mass shootings.
Murphy said he told colleagues that mental illness was the common link in similar tragic incidents and "we have to stop pretending it doesn't exist. We need to understand what it is that triggers changes in someone."
"I see it as the center of the issue. Get mental illness out of the shadows."
Associated Press writer Julie Pace contributed.