Newtown - Nelba Marquez-Greene put her two children on the school bus on the morning of Dec. 14. Only one came home.

Nicole Hockley still finds herself reaching for her son's hand in parking lots or expects "him to crawl into my bed for early-morning cuddles before school."

"It's so hard to believe he's gone," she said.

The grieving mothers and other parents and family members of those killed in the Sandy Hook Elementary School massacre gathered here at a news conference Monday to help begin a campaign aimed at preventing the kind of bloodshed that has turned their town into a national symbol of grief.

In some of their first public statements since the shooting that killed 20 children and six staff members at the school, the families of 11 of the children and adult victims called for a national dialogue on issues of mental health, school safety and what their nonprofit organization, called Sandy Hook Promise, described as "gun responsibility."

The gathering came as President Barack Obama and Vice President Joe Biden prepare to unveil gun-control proposals as soon as today that are expected to call for a ban on the kind of assault weapon and high-capacity ammunition magazines used by Adam Lanza in the Newtown shooting.

But perhaps foreshadowing the debates to come in Washington, group members declined to offer support for any specific measures, saying they needed time to educate themselves on the issues and emphasizing that the debate must be broader than gun control.

"It's only been 30 days, and for the past 30 days we've really been looking inward and supporting our community," said Tim Makris, a founder of the group whose fourth-grade son at the school was not hurt.

"We love the focus of the president," he added, "and we love that the vice president reached out recently to talk directly to the families that chose to meet with him. But we don't have an immediate response right now."

Tom Bittman, another founder who had three children attend the school, said that many group members were gun owners.

"We hunt; we target shoot," he said. "We protect our homes. We're collectors. We teach our sons and daughters how to use guns safely. We're not afraid of a national conversation in our community and in Congress about responsibility and accountability. We know there are millions of people in this nation who agree with us."