More than 140 Mexican prisoners have escaped from a jail near the US border, local media reports say.
The prisoners broke out of the jail in Nuevo Laredo late on Thursday or early on Friday morning.
Security guards are being questioned on suspicion of helping the inmates flee, and prison director has gone missing.
Mexico's prison system is struggling to cope with an influx of violent offenders arrested in the government's campaign against drugs cartels.
Nuevo Laredo in Tamaulipas state lies just across the border from Laredo, Texas.
Tamaulipas security chief Antonio Garza said the break-out - one of the largest in the country's history - went unnoticed until guards carried out a routine head count.
Officials believe the inmates had escaped through one of the prison's back gates, with the connivance of one or more security guards.
Mr Garza said all the guards were being considered suspects in helping the prisoners to flee, and the prison director had been "listed as missing".
Security forces in the area have launched a massive search, while troops and federal police have been deployed to guard the prison.
It usually holds about 1,000 inmates, many of them convicted of federal crimes such as drugs and kidnapping.
This is the second mass prison break in northern Mexico in recent months.
In September, 85 prisoners escaped from a jail in the nearby border city of Reynosa, also in Tamaulipas.
The northern border region is the scene of rising lawlessness as the cartels fight the security forces and battle each other for control of smuggling routes into the US.
The drug cartels' capacity for violence and ability to pay huge bribes gives them considerable power to subvert the prison system and get their people out.
In July, Mexican prosecutors accused guards at a prison in Durango state of allowing inmates to leave the jail to carry out contract killings for the cartels, and of lending them weapons and vehicles.