The timing and origin of the alleged incident remain unclear.
The US Federal Bureau of Investigation (FBI) has said it has been "searching for malicious cyber activity" on its internal computer network over the past few days.
According to FBI sources, the incident is related to a computer system located in the FBI office in New York, which was used to investigate the distribution of child pornography. It is not yet known how the attackers managed to gain access to the network.
"The FBI is aware of the incident and is working to obtain additional information," the statement said. “This is a single incident that was localized. The FBI is not commenting on the incident as the investigation continues."