Emre Encok had just boarded the bus at the Ahui Mesut Boulevard station in Ankara when he began shooting at co-workers, Today's Zaman reported.
Witnesses told police Encok yelled, "How could you do that to me?" before opening fire.
Initial reports indicate he accused colleagues of slander.
Police identified the dead guards as Ali Duran, Celebi Aygun and Mustafa Halici.
Female guard Seda Duran, the wife of Ali Duran, was seriously injured and taken to a local hospital.
Encok was arrested and police have opened an investigation.
All five guards worked at Sincan Prison.
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