Lennarth Adel of Tempest Security told the Aftonbladet newspaper a cleaner discovered the woman sleeping in a pool of her own vomit, urine and feces around 6 a.m. Tuesday at the Riksdag, TheLocal.se reported Wednesday.
"Something went really wrong here," Adel said.
Adel said the woman may have been able to enter due to a technical problem with security doors or she may have been let inside the building by someone else.
John Arentoft, head of the Riksdag security team, said the woman could have also been a lost tourist.
"This isn't the first time someone has been found on our premises and shown the door," Arentoft told the TT news agency. "Sometimes we find visitors who have got lost or have been separated from their group. We're investigating now how the woman got herself in here, and whether she was part of a visiting group, for example."
The woman was escorted from the building by police.