According to Fletcher, Valbuena and Castillo played in a Venezuelan league game Thursday night.
The club tweeted, "We are living a disgusting tragedy in Cardenales de Lara".
"We are saddened by the passing of former Cub Luis Valbuena", the Cubs tweeted Friday morning.
High speed led to the crash early Friday in Yaracuy state, a report by transit police says.
Valbuena was in the major leagues as recently as August, when he was released by the Los Angeles Angels.
Castillo, also an infielder, played five seasons in the majors for the Pirates, Giants and Astros from 2004-08.
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Major League Baseball tweeted late Thursday the 33-year-old Valbuena and 37-year-old Castillo died. "Our thoughts and prayers are with their loved ones during this time of grief".
"During his two seasons in Houston, Luis was an extremely popular player with his teammates and the Astros staff".
Primarily a third baseman who also saw action at the other three infield positions, Valbuena hit a career-high 25 homers for the Houston Astros in 2015.
The crash occurred after Valbuena and Castillo left the University Stadium at Caracas after playing a game in the Venezuelan Professional Baseball League.
The vehicle crash that claimed the lives of two former Major League Baseball players on Thursday in Venezuela may have been a criminal act, and officials are reportedly looking into whether a gang of thieves engineered the wreck in order to rob the pair.