Officials said they have received numerous tips but no confirmed sightings of Hannah Anderson, a missing 16-year-old girl who was kidnapped by a longtime family friend.
The case has the subject of the California's first Amber Alert text alert on Monday and Tuesday.
"We think she's in grave danger," Lt. Glenn Giannantonio told Fox 5 San Diego. "Realistically we don't know where they're going. We're keeping a wide net and hopefully we'll find them. We're looking everywhere."
Friends gathered at the vigil at El Capitan High School wearing pink, Hannah's favorite color. They also released blue balloons in memory of her 8-year-old brother, Ethan, who is believed dead.
James Lee DiMaggio is suspected of killing the children's mother, Christina Anderson, 44, of Lakeside and then kidnapping one or both of the children.
Christina Anderson's body was found Sunday night in the burning rubble of a house and garage in the rural community of Boulevard in eastern San Diego County, authorities said.
The Sheriff's Department said an autopsy on the child's remains was performed Tuesday but that the autopsy results are sealed.
Authorities believe DiMaggio is driving a blue Nissan Versa, California license plate 6WCU986. A statewide Amber Alert remains in effect.
At the vigil, Hannah Anderson's friend Haley Wilcock told Fox 5 San Diego: "We have hope still that she's still out there. And we hope that she somehow hears about this and knows that we're all here for her and praying for her."
Hannah's father, Brett Anderson, also addressed the crowd.
"Hannah will come back and Ethan and Tina, I love you both," said Anderson. "We all love you. I know everybody's here for support of all three of them and I want to thank you all. God bless."
Hannah's cousin pleaded with the suspect, James DiMaggio, to bring Hannah home safe.
"Please bring Hannah home to us," she said. "We all miss you. Hannah we love you so much."
At a Tuesday afternoon news conference, Brett Anderson said, "I'm begging you to let my daughter go. You've taken everything else."
DiMaggio had been considered a close family friend and the children had considered him like an uncle. Authorities aren't releasing a motive.