Hartford — Connecticut lawmakers announced a deal Monday on what they called some of the toughest gun laws in the country that were proposed after the December mass shooting in the state, including a ban on new high-capacity ammunition magazines like the ones used in the massacre that left 20 children and six educators dead.

The proposal also called for background checks for private gun sales and a new registry for existing magazines that carry 10 or more bullets, something of a compromise for parents of Newtown victims who had wanted an outright ban on them, while legislators had proposed grandfathering them into the law.

The package also creates what lawmakers said is the nation's first statewide dangerous weapon offender registry, immediate universal background checks for all firearms sales and expansion of Connecticut's assault weapons ban.

A new state-issued eligibility certificate would also be needed to purchase any rifle, shotgun or ammunition under the legislation. To get the certificate, a buyer would need to be fingerprinted, take a firearms training course and undergo a national criminal background check and involuntary commitment or voluntary admission check.

The deal is "the most comprehensive package in the country because of its breadth," said Senate Minority Leader John McKinney, a Fairfield Republican whose district includes Newtown.

McKinney said people tend to focus on assault weapons and high-capacity magazines, but he said "there's a lot here underneath the surface" addressing mental health, school security and other issues.

The proposal was revealed to rank-and-file lawmakers Monday after weeks of negotiations among legislative leaders. A vote was expected later this week in the Legislature, where Democrats control both chambers, making passage all but assured. The bill would then be sent to Gov. Dannel P. Malloy, who has helped lead efforts to strengthen the state's gun laws.

Connecticut is sending a message to Washington and the rest of the country "this is the way to get this job done," said House Speaker Brendan Sharkey, a Democrat from Hamden. Both Democratic and Republican leaders were expected to support the proposal, which had been in the works for about a month.

The shooting reignited the gun debate in the country and led to calls for increased gun control legislation on the federal and state levels. While some other states, including New York, have strengthened their gun laws since the shooting, momentum has stalled in Congress, whose members were urged by President Barack Obama last week not to forget the shooting and to capitalize on the best change in years to stem gun violence.

The gunman in Newtown blasted his way into Sandy Hook Elementary School and fired off 154 shots with a Bushmaster .223-caliber rifle within five minutes. He went through six 30-round magazines, though half were not completely empty, and police said he had three other 30-round magazines in addition to one in the rifle. He gunned down 26 people, then shot himself to death with a handgun.

Six relatives of Newtown victims visited the Capitol on Monday, asking lawmakers to include a ban on existing high-capacity magazines. Some handed out cards with photographs of their slain children. They delivered a letter signed by 24 relatives that demanded that legislators include existing large-capacity ammunition magazines in the ban on the sale of magazines that carry 10 or more bullets.

Allowing such large-capacity magazines to remain in the hands of gun owners would leave a gaping loophole in the law, said Mark Barden, whose 7-year-old son, Daniel, was killed in the shooting.

"It doesn't prevent someone from going out of the state to purchase them and then bring them back. There's no way to track when they were purchased, so they can say, "I had this before,"' Barden said. "So it's a big loophole."

Jake McGuigan, a spokesman for the National Shooting Sports Foundation, which is based in Newtown, said he wouldn't comment on the proposal until he saw it in the writing, but he questioned the mechanics of a registry for magazines.

"How will they register a magazine? It seems a little weird," he said.