PLEASANTON — A teenage driver originally accused of vehicular manslaughter now faces a murder charge in the death of a bicyclist, partly because prosecutors say he bragged on Twitter about driving dangerously.

His case is part of a growing trend of social media posts being used as evidence against suspects, authorities said Friday.

Cody Hall, 18, was originally charged with felony vehicular manslaughter in the death of 58-year-old Diana Hersevoort, of Dublin. But this week, prosecutors raised that felony count to murder, in part, they said, due to his Twitter feed.

Hall was allegedly traveling at 83 mph — more than double the speed limit — on Foothill Road, near Golden Eagle Way, in Pleasanton when he lost control and struck Hersevoort and her husband, who suffered a broken bone, authorities said.

Prosecutors did not describe specific posts, just the Twitter feed in general, in increasing the charge. An attorney for Hall could not be reached Thursday.

As suspects feel compelled to post their misdeeds online for audiences to see, investigators have taken advantage, using the online quasi-confessions to bolster their cases, Bay Area prosecutors said.

In San Francisco, a cyclist in March fatally struck a 71-year-old pedestrian in a crosswalk after speeding through three red lights in the Castro District. Chris Bucchere, who eventually pleaded guilty to felony vehicular manslaughter, received a stiffer charge after he posted his explanation of the crash on a cycling group’s website.

Hall is scheduled to appear in Alameda County Superior Court on Monday for a bail motion hearing.

Authorities said they considered Hall’s Twitter feed, his driving history and his speeding on the day of the fatal crash when deciding to upgrade the charge to murder.

More commonly, murder charges have been filed in fatal crashes against suspects who recklessly flee police or against intoxicated drivers with prior DUI convictions, said Brian Welch, supervisor of the homicide unit of the Santa Clara County District Attorney’s Office. Prosecutors must show “implied malice,” meaning the defendant engaged in an intentional and unlawful act and did so with disregard for human life, to prove a murder charge, he said.

In this case, it appears the Twitter feed will serve as a “pre-offense statement,” not unlike an email, text message or handwritten note to bolster the prosecution’s theory of implied malice, Welch said.

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

Contra Costa County prosecutor Derek Butts said that verification takes some common-sense gumshoe work, often made easier when defendants send photos of themselves or reveal details only known by them.

Butts prosecuted a Richmond gang murder case in which he used subpoenas to get MySpace messages posted by various gang members in which they discussed criminal activity openly with each other and taunted other gang members. One MySpace account revealed an email from one gang member to another telling him to watch the nightly news because it was featuring a shooting he committed.

“It affects a case greatly,” Butts said. “It can be damaging evidence.”

So why are criminals compelled to post their own malevolence? It depends, said Butts, but for some it’s like a terrorist group claiming responsibility for an act of violence.

“When you’re a part of a gang, it’s important for people to know how bad a guy you are,” Butts said. “You don’t secretly whack a gang member, because what’s the point of that? They want people to know they did it so people won’t mess with their neighborhood.”

Contact Matthias Gafni at 925-952-5026. Follow him at Twitter.com/mgafni.