Police have arrested three men suspected of throwing banned items into HMP Wealstun at Thorp Arch.
Guards at the men's prison near Wetherby discovered packages containing pills and mobile phones that had been thrown over a fence on Sunday afternoon.
Three men from Bradford were later arrested when their car was stopped on Easterly Road in Harehills.
West Yorkshire Police said:
"At 3.05pm yesterday (30/12) police were contacted by staff at HMP Wealstun, Thorp Arch, reporting an incident where packages containing mobile phones and pills had been thrown over a fence.
"A vehicle linked to the incident was later stopped by officers in Easterly Road, Harehills, and three men, aged 20, 23 and 26, from Bradford, were arrested on suspicion of throwing an article into a prison.
"They were subsequently released under investigation while enquiries into the incident continue.
"Anyone who witnessed the incident or saw anything suspicious is asked to contact officers via 101 quoting crime reference number 13180651983."