Juan Mendoza made the comments after the women, both 20, made a second appearance before a judge at Sarita Colonia del Callao jail, in Callao, near Lima. He did not give details of the private hearing but suggested the women's confessions had not yet satisfied prosecutors.
Last week Reid, from Lenzie, and McCollum, from Dungannon, Co Tyrone, pleaded guilty to drug smuggling. They had hoped the behind-closed-doors admission would be enough to secure a shorter sentence.
But prosecutors have demanded more information before accepting their admissions of guilt, which the women hope will bring their jail time down to six years and eight months.
Mr Mendoza would not discuss whether the women had explained to them how they got the cocaine, or who it was from.
McCollum and Reid initially claimed they were kidnapped, held at gunpoint and forced to board a flight from Lima to Spain with 24lb of cocaine in food packets hidden inside their luggage when they were arrested.
An Irish priest said the pair were in great health as they awaited their fate.
Father Maurice Foley visited last Saturday.
"(They are) brilliant. Very, very well," he said.
"They weren't in a cell. They were out in a wide open space sitting at a table with a parasol, they were talking and drinking coffee.
"As well as that they had telephone communication and they could use it for calling home. I thought they were in great form actually."
He said the prosecutors wanted the women to hand over names and addresses of the gang who coerced them into the trafficking.
"They don't have that. All they have is the name of a cockney Englishman by the name of Jake or Joe and that's no good," he said.
"I think they just don't have that information themselves. My firm belief is that they were conned."