Officials said some bandits place or throw objects on highways to force vehicles to stop or crash so they can rob the occupants. Heredia said four people have been detained after being found with property of the athletes.
Valbuena and Castillo were teammates on the Cardenales de Lara team in the Venezuelan winter league and were returning from a game in the capital when the crash occurred en route to the city of Barquisimeto.
Third baseman Carlos Rivero was in the car and survived, according to the website BeisbolPlay.
Valbuena, an 11-year major league veteran, hit .199 with 9 home runs and 33 RBIs in 96 games this year for the Los Angeles Angels, who released him Aug. 7.
"I will miss Luis' banter, smile, genuine love for his teammates and, of course, the bat flips," AJ Hinch, who managed Valbuena with the Houston Astros in 2015-16, said in a statement. "He was a beloved person whether he was on our team or across the field."
Valbuena hit .226 with 114 home runs over 11 big league seasons with the Angels, Seattle Mariners, Cleveland Indians, Chicago Cubs and Astros.
Castillo played five seasons with the Pittsburgh Pirates, San Francisco Giants and Astros. He had a .254 average with 39 home runs.
Venezuelan President Nicolas Maduro said in a tweet, "I join in the mourning that has overwhelmed the Venezuelan baseball family and all of its fans."
Baseball commissioner Rob Manfred issued a statement calling it "a very sad day for our sport.
"It is clear by all the stories today that they loved baseball and made an impact on their teammates and the clubs they represented," Manfred said, expressing "my deepest condolences to their families, friends and fans, particularly those in their native Venezuela."
By late Friday afternoon, the coffins with the players' bodies had arrived in Antonio Herrera Gutierrez stadium in Barquisimeto, where they were received amid tears by their teammates. A mass was held to bid them goodbye.
The Venezuelan Professional Baseball League announced that all three scheduled games Friday were canceled and said flags will fly at half-staff in all stadiums for three days.
League president Juan Jose Avila told Union Radio that he was evaluating the possibility of forbidding players to travel in private cars to move between games, saying if the players had been on the team bus, "nothing would have happened to them."
Venezuelan teams tend to travel in buses protected by security forces.
Dozens of Venezuelans waited in line outside a chapel in the state of Lara on Saturday, hoping to bid farewell to Valbuena. Castillo was moved earlier in the morning to a different central-west state.
Retired Cardenales player Robert Perez and Philadelphia Phillies shortstop Asdrubal Cabrera were both in attendance.
Rivero, the crash's lone survivor, visited the chapel wearing dark sunglasses and bearing a small bruise on his forehead.
Information from The Associated Press was used in this report.