"The fact that this problem is complex can no longer be an excuse for doing nothing," he said. "The fact that we can't prevent every act of violence doesn't mean we can't steadily reduce the violence and prevent the very worst violence."
Obama highlighted suggestions to restrict gun sales to criminals and the mentally ill and to improve access to mental health care.
Holder was to travel later in the day to Connecticut to meet with law enforcement officials and first responders, a Justice Department official said.
Since the shootings, a number of conservative Democrats and some Republicans who have supported gun rights have said they are open to discussing the issue.
Sen. Dianne Feinstein, D-California, said she will introduce legislation to reinstate the assault weapons ban that expired in 2004. The White House has said that the president supports that effort.
More than 195,000 people have signed an online White House petition supporting new gun-control legislation.
The gun industry itself has been largely silent on the issue; the National Rifle Association said Tuesday it would offer "meaningful contributions to help make sure this never happens again." The group has scheduled a news conference for Friday morning.
Gun control advocates say they believe the killings have so shocked the nation's conscience that change may be possible.
"I think that we are at a historic moment," said Sen. Richard Blumenthal, D-Connecticut.
In Ontario, Premier Dalton McGuinty made $10 million available to pay for security upgrades to establish a locked-door policy at 4,000 of the province's elementary schools.
"We're not going to brick up these windows; that would be unreasonable. But I believe there is a reasonable expectation by parents that when their kids go to elementary school in Ontario that we will have a locked-door policy in place," he said.