The 42-year-old woman turned up with her nearly one-year-old baby and 11-year-old daughter at the office she shared with her husband, a well known landscape architect, on Wednesday evening, a local police spokeswoman said.
"We were alerted at just before 8pm that three people fell from the 13th floor, from an office." she added.
Investigators have not ruled out any hypothesis, the spokeswoman said, while several newspapers citing police sources said officers suspect the woman may have committed suicide after throwing her daughters from the office.
The bodies of the baby and the girl fell in front of a man who was taking a drink in an outdoor terrace, daily newspaper El Pais reported. Girona mayor Carles Puigdemont led city workers and local residents in a minute of silence at noon in front of the town hall to mark the deaths.
"The city has been dealt a blow," he told reporters.
The woman had worked for the town hall as an engineer, said a town hall employee who described her as "very dynamic".
She had another child with her husband, who has won several awards for his work as a landscape architect.