An 18-year-old accused of killing a bicyclist with his car has had a vehicular manslaughter charge upgraded to murder in part because he boasted about speeding on Twitter, prosecutors said Thursday.

Cody Hall, of Pleasanton, was being held without bail after he was charged Wednesday with the murder of 58-year-old Diana Hersevoort, the San Francisco Chronicle and Oakland Tribune reported.

Hall was going more than 80 mph in a 40 mph zone when he hit Hersevoort and her husband along a busy boulevard in Dublin on June 9, prosecutors allege. Hersevoort's husband only broke an arm, but she was killed.

An analysis of Hall's driving record, along with Twitter posts in which he discussed how fast he liked to drive, persuaded prosecutors to change the charge to murder, the Alameda County district attorney's office told the Chronicle.

Prosecutors did not immediately reply to messages left by The Associated Press, and a Twitter account apparently belonging to Hall is restricted from public view.

Brian Welch, a supervisor of the homicide unit at the Santa Clara County district attorney's office who is not involved in this case, told the Tribune that in most circumstances, fatal crashes result in murder charges when the suspect was recklessly fleeing police or was a drunken driver with previous convictions, not because of something like a Twitter feed.

Welch said in this case it is likely the tweets will serve as what's called a "pre-offense statement," often an email, handwritten note or text message, used to bolster prosecutors' attempt to prove malice.

"The challenge in these situations is proving that your defendant is the person who posted the statement," he said.

Neither the newspapers nor The Associated Press could reach an attorney for Hall for comment.