US investigators say it could be a "state-sponsored attack".
The FBI has started a probe into reported cyber attack on Virginia-based company US Investigations Services which was working as contractor with US Department of Homeland Security (DHS) to check backgrounds of officials.
The breach was detected recently and the company is anticipating that the hacker has probably stolen personal information, according to the Washington Post.
DHS spokesman Peter Boogaard was quoted by the Washington Post as saying: "A multiagency cyber-response team is working with the company to identify the scope of the intrusion, and the FBI has launched a federal investigation into this issue."
Following the incident, US Department of Homeland Security has suspended its work with the company to prevent compromise of additional information until the investigation is completed.
US Investigations Services said in a statement: "Experts who have reviewed the facts gathered to-date believe it has all the markings of a state-sponsored attack."
"We are working closely with federal law enforcement authorities and have retained an independent computer forensics investigations firm to determine the precise nature and extent of any unlawful entry into our network," the statement added.
"We are working collaboratively with OPM and DHS to resolve this matter quickly and look forward to resuming service on all our contracts with them as soon as possible."
"We will support the authorities in the investigation and any prosecution of those determined to be responsible for this criminal attack."