Sources close to the investigation into the murders of a married Washington, D.C., couple, their 10-year-old son and their housekeeper reportedly believe the couple was bound and held captive for at least one night before they were brutally killed and their mansion set on fire.

Savvas Savopoulos, 46, his wife, Amy Savopoulos, 47, their son, Philip, and housekeeper Veralicia Figueroa, 57, were found dead Thursday afternoon.

Sources told NBC Washington that authorities believe more than one person was involved in the murders and it’s likely that the killers gained access to the home Wednesday and kept the victims bound for a day until Savvas gave them what they were looking for.

Neither of the two security cameras at the house picked up any activity related to the incident, sources told Fox 5. Investigators also told the station that there were no signs of forced entry.

The couple reportedly called and texted another housekeeper to not come to work on the day of the killings.

Over the weekend, D.C. police released a video of a person of interest in connection to the murders, which took place just blocks away from Vice President Joe Biden’s official residence.

Authorities said at least three of the victims suffered stab wounds or blunt-force injuries.