Congressional Democrats challenged Republican lawmakers by reviving legislation that would deny the sale of firearms to those on terrorism watch lists, which had failed to pass in December.

The Democrats said they will try to attach the revived legislation to the Commerce, Justice and Science spending bill that will be on the Senate floor this week, according to the web site Politico.

New York Senator Charles Schumer, a supporter of the legislation, said that Democrats believe they will "do better" this time in getting Republicans to support it. "Circumstances are going to force them to see the light," he told Politico.

Julian Mortenson, a professor of law at the University of Michigan, said that gun control advocates expect each mass shooting to be the catalyst for significant reform, but it has not happened.

"So I don't see signs of the fundamental political dynamic changing, and until we see some of that -- and it's hard to tell when that will happen, perhaps at some point it will -- I'm not sure we should expect the outcomes to be any differently politically," he said.

The Brady Campaign to Prevent Gun Violence advocacy group said in a statement released on Sunday when the Orlando shootings took place that it is "far too easy" for those who are intent on causing harm to access guns.

"We owe it to the people of Orlando -- and to all of the 90 Americans killed each day by guns -- to do everything in our power to prevent this kind of violence," said Dan Gross, president of the organization.

Obama said on Monday that "it appears that the shooter was inspired by various extremist information" online; he added that "all those materials" are now being scrutinized and "exploited''.

Investigators said they are operating under the theory that the attack was inspired by ISIS. But Mateen, who had been interviewed twice by the FBI and once was on a terrorist watch list, had previously expressed solidarity with Islamist groups that oppose Islamic State, FBI Director James Comey said on Monday, adding "confusion" about his inspiration for the attack, the Los Angeles Times reported.

Mateen's ex-wife, Sitora Yusufiy, said outside of her home in Boulder, Colorado, on Sunday that their marriage lasted four months and that she had left their Florida home and filed for divorce. She said that Mateen had been abusive, and at times displayed erratic behavior. "There were definitely moments when he'd express his intolerance toward homosexuals," she said.

Contact the writer at [email protected].