Sixteen gang members and their associates from East Palo Alto and Menlo Park were indicted by a grand jury for murder, attempted murder and other crimes following a year-long investigation by regional law enforcement agencies, the San Mateo County District Attorney's Office announced Friday.

They were connected with three street gangs and committed crimes up and down the Peninsula from East Palo Alto to San Francisco, according to the district attorney's office.

In addition to murder and attempted murder, the defendants were indicted for conspiracy to murder, dissuasion of witnesses and attempted armed robbery.

Chief Deputy District Attorney Karen Guidotti wouldn't specify when the grand jury indictments occurred other than to say "recently." That information and other details will be presented at a press conference scheduled for Monday morning in front of the San Mateo County Courthouse in Redwood City, she said.

District Attorney Steve Wagstaffe is expected address the media then, along with Sheriff Greg Munks, East Palo Alto Police Chief Lee Violett and other law enforcement officials, according to the announcement.

The arrests stemmed from a multi-agency effort named Operation Sunny Day that was the "largest and most extensive investigation of its kind in San Mateo County," according to a district attorney's office announcement. Law enforcement agencies from San Mateo, Santa Clara and San Francisco counties participated.

East Palo Alto and eastern Menlo Park have been plagued by gang-related crime for years. More than a dozen shootings and several slayings that occurred there over a six-month period in 2012 were attributed to a violent feud between two rival street gangs. And in 2009, local and federal law enforcement agents arrested several dozen members of the East Palo Alto and Menlo Park-based Taliban gang.

Menlo Park Mayor Ray Mueller, who had not heard about Operation Sunny Day or the arrests, said he was nonetheless pleased by the outcome.

"I'm thrilled," Mueller said Friday. "I think it's an appropriate name. The sooner that we can rid crime from our streets and provide a community that's safe for children to play and prosperity to thrive, that will be a sunny day for everyone."

Email Bonnie Eslinger at beslinger@dailynewsgroup.com; follow her at twitter.com/bonnieeslinger.