An intensive search began after Allison, 15, was reported missing by her family on Monday.
She was last seen at about 8 a.m. Monday on her bike in front of San Ramon Valley High School in Danville. Her bike was later found near the bridge.
Danville police have said that Allison was considered at-risk and that no criminal activity is suspected.
Additional details discovered on Tuesday have led investigators to believe Allison took her life at the bridge on Monday morning, CHP Capt. Amy Mangan said today.
Mangan said there are cameras on both sides of the bridge, but wouldn't comment if officers had footage of any incident related to Allison.