Overcrowding is considered the probable cause: the vessel, licensed to carry 80 passengers on its tours of Lake Banyoles, was carrying 141.
Officials said care would be taken not to destroy any clues as to why the boat, the Oca, went down on Thursday .
Magistrates in Gerona were questioning the owner and the pilot of the boat, run by a family company, La Carpa, to establish whether overcrowding was the cause.
The two men remained at liberty. Inquiries were under way yesterday to establish if the boat had a leak or a mechanical fault.
A newspaper said the owner did not have the necessary licence. The Mayor of Banyoles, Joan Solana, said the owners had a licence, "but the courts will establish whether or not it was the right one".
Twenty survivors were expected to leave hospital yesterday but a 67-year- old woman was in a serious condition in a Barcelona hospital.
Some of the elderly French tourists on a package holidaytried to recover from the trauma of their ordeal, but were stricken with grief and anger. "I don't really know what is going to happen now and I don't really care," said one.
Dolores Portella, owner of the Hotel Mirallac, overlooking the lake, said the tragedy was catastrophic for her livelihood. "I've had French families phoning up with cancellations, and it'll snowball next week."
She looked out at the Anna, sister boat to the Oca, moored by the lakeside. "It'll be a long time before that makes another trip."Reuse content