An official in the state prosecutors' office, who was not authorised to be quoted by name, yesterday said the older daughter had not yet been detained because police were waiting for a formal arrest warrant.
The alleged culprit has been identified as Anastasia Lechtchenko, but the Russian Embassy in Mexico City could not immediately confirm that name.
The prosecutors' official said that the young woman told investigators she planned the crime and committed it without accomplices, but that the circumstances of the killings were still under investigation.
In a statement Thursday, prosecutors had said that two people were being questioned and that "the family environment could have been the cause."
Yesterday, the prosecutors' official described a rough, rebellious relationship between the mother and her older daughter for at least the last five years. The young woman left home at least three times and had allegedly been involved with people who were using drugs, the official said.