Mexican security forces have scrambled to save face and recapture drug kingpin Joaquin "El Chapo" Guzman as authorities investigated whether guards helped him escape prison through a tunnel under his cell.
For the second time in 14 years, the head of the powerful Sinaloa drug cartel managed to flee a maximum-security prison, dealing an embarrassing setback to President Enrique Pena Nieto.
Troops and police manned checkpoints and hunted for Guzman after he vanished late Saturday from the Altiplano prison 90 kilometres west of Mexico City, after just 17 months behind bars.
Prosecutors have questioned more than 50 people so far.
A federal official said 32 prison employees of various rank, including the warden, spent the night at the anti-organised crime unit of the attorney general's office, but no charges have yet been filed.
The guards in charge of the capo's cell and those who monitored the surveillance cameras that look into the room were among those interrogated.
"They are making statements, with the assistance of lawyers and human rights personnel," the official from the prosecutor's office told AFP on condition of anonymity.
Two of Guzman's lawyers were questioned and anybody else who visited him during his incarceration is being sought, the official said.
Authorities had already investigated a strange prison visit to Guzman in March when a woman managed to see him by using a fake ID to enter the jail.
Another person being sought for questioning is the owner of the property where Guzman's 1.5km tunnel ended.
Guzman, 58, who nurtured a Robin Hood image in his northwestern state of Sinaloa while running the most powerful and one of the most ruthless cartels in Mexico, was able to slip out even though surveillance cameras were trained on his cell.
He went into his private shower and after he failed to come out, guards found a hole 10 metres deep with a ladder inside.
The gap led to a sophisticated tunnel with a ventilation and light system that ended inside a grey brick building on a hill surrounded by pastures in central Mexico State.
A huge water pipeline project is under construction around the prison, which could explain why the tunnel's construction went unnoticed.
The US State Department said Guzman's "swift recapture by Mexican authorities is a priority for both the Mexican and the US governments".
Guzman's first escape was in 2001, when he slipped past authorities by hiding in a laundry cart in western Jalisco state.
Marines recaptured him in February 2014 in a pre-dawn raid at a condo in Mazatlan, a Pacific resort in Sinaloa state, with the DEA's help.