We want the Sandy Hook school shootings to be recalled as the turning point where we brought our community and communities across the nation together and set a real course for change," said group co-founder Tom Bittman.

While the Sandy Hook group did not offer specific remedies, mayors and governors in favor of tighter gun restrictions lobbied for a series of them Monday.

New York Mayor Michael Bloomberg addressed a summit on gun violence at Johns Hopkins University in Baltimore and argued for greater federal gun control, including background checks for all purchases and a federal crackdown on trafficking.

"Every state in the union has citizens killed by guns coming from another state and every state is powerless to stop the mayhem," Bloomberg said.

Bloomberg is co-chairman of Mayors Against Illegal Guns, whose members spoke out Monday in cities including Portland, Maine; Fort Wayne, Ind.; and Santa Fe, N.M.