An intensive manhunt along Mexico's northern border has failed to turn up any of the more than 140 prisoners who escaped from a federal prison.

The inmates apparently walked out unchallenged Friday through the main gate of the prison in the northeastern city of Nuevo Laredo.

Tamaulipas state security chief Antonio Garza told local radio that the prison's director was reported as missing, along with the inmates.

Prison staff are under suspicion of being involved in the escape, the latest in a series of such incidents in Tamaulipas this year.

Garza said the vehicle entrance was used as the escape route, "presumably with the assistance of the prison staff."

The last mass escape in Tamaulipas occurred in September when 85 inmates broke out of a prison in the northern border city of Reynosa.

The state has been plagued by a steady wave of violence tied to turf battles between the Gulf and Zetas drug gangs, but it was unclear whether members of those groups were among the inmates who escaped on Friday.