Mexican authorities have dropped their investigation into claims six British cavers rescued from a flooded cavern were illegally looking for uranium.
The cavers were transferred to the Immigration Institute
But the men, who were on a military training exercise, are still being held over possible visa irregularities.
The attorney general decided not to proceed with the investigation after seeing the men on Saturday, a Ministry of Defence spokeswoman said.
Immigration officials have until Monday morning local time to question the men.
Head of the British Army, General Sir Michael Jackson, earlier told BBC's Breakfast with Frost that the detention was simply the result of a "great misunderstanding".
"This was a joint service adventure training expedition. We do a lot of that to bolster initiative and courage," Sir Michael said.
"It seems to me that their purpose was to explore these
caves further than they had already been explored," he added.
The men were arrested on suspicion of entering Mexico illegally after their rescue from the flooded cavern near Cuetzalan.
But local media reports had claimed the men were searching for uranium - an allegation denied by the MoD.
Mexican authorities transferred the expedition members to the federal attorney general's office on Saturday for an evening of questioning.
An MoD spokeswoman said the attorney general released them without charge into the custody of immigration officials.
In a statement, the attorney general's office said that immigration authorities were clear to process the group's documents for departure.
From there they were moved to the Immigration Institute where they spent the night.
It is now up to immigration officials to assess whether they broke their tourist visas.
Scuba divers brought the men to the surface one-by-one
If they are found guilty of breaching immigration rules they could be deported, face 18 months in jail, be fined up to £180 or prohibited from returning to Mexico for a specified period.
They were due to fly back to Britain on Saturday morning, but missed that flight because of their continuing detention.
Mexico's President Vicente Fox intervened in the affair, saying he was unhappy with explanations for their activity in the caves, where they became trapped by floods.
Mr Fox said Britain had not clarified answers to questions about the activities of the cavers.
The Mexican government is upset it was not told in advance of the presence of the expedition.
The men were members of a 13-strong team from the Combined Services Caving Association (CSCA), and four are serving in the British military.
They are Sergeant John Roe of the RAF, Navy
warrant officer Charles Milton and Sergeant Chris Mitchell and Captain Toby Hamnett from the Army.
The other two were retired Army Major Jonathan Sims and civilian caver Simon Cornhill.
The CSCA said in a statement on its website that the caving trip had been planned for three years.
The cave exploration, which began last Monday, was due to last 36 hours but the group got stuck after heavy rain raised the water level in the cave.
They declined local offers of rescue, instead preferring to wait for two specialist British divers who flew to Mexico to escort them out.