Brown, who wasn't a regular smoker, purchased the device on January 27 while on the way to the bank, the grandmother told the Fort Worth Star-Telegram.
The pen's battery blew up when William Brown tried to use it, sending shards of metal into his face and neck and severing an artery.
Local media reports indicate that after the explosion, William was able to get out of the vehicle to search for help; however, he ultimately collapsed near the trunk of the auto. He was taken by ambulance to John Peter Smith Hospital (JPS) in Fort Worth.
The Tarrant County Medical Examiner's office says shrapnel from the explosion severed his left carotid artery and peppered his skull. He was just weeks away from his 25th birthday. It's a waste of the things he could have accomplished, ' she told the Fort Worth Star-Telegram.
"It just all seems so unreal", she told the news outlet. "And that's what they should have taken out as soon as he got to the hospital, and they made a decision to wait until Monday or Tuesday".
He attempted to use the pen in the auto when the battery exploded with enough force to melt plastic in the vehicle and fling metal debris into Mr Brown's face.
In a statement, the hospital said it could not discuss case specifics due to privacy laws, but added it "will take family concerns seriously as we review all that transpired". His family said a vape pen exploded while he was trying to use it.
'He had a future ahead of him... a life ahead of him'.
He had never used a vape pen before and was using it in his grandmother's vehicle on January 27.
Brown is the second person in the USA known to have died due to an exploding e-cigarette. That's what the investigator said he needed.
In 2017, the U.S. Fire Administration reported 133 acute injuries from e-cigarettes, vaporizers and other similar devices between 2009 and 2016, with most fires and explosions occurring while the e-cigarette was in a pocket or being used.
People who smoke e-cigarettes are also 71 percent more likely to have a stroke, the study claims.