The operator of the Nasdaq Stock Market said it found "suspicious files" on its U.S. computer servers and determined that hackers could have affected one of its Internet-based client applications.

According to an AP report, there was no evidence the hackers accessed or acquired customer information or that any of parent company Nasdaq OMX Group Inc's trading platforms were compromised.

 The report says that the FBI and outside forensic firms helped conduct the investigation, though Nasdaq OMX did not say when it was launched or when the suspicious files were found. The files, found in a Web application called Directors Desk, have been removed, the report said.

"We continue to evaluate and enhance our advanced security controls to respond to the ever increasing global cyber threat and continue to devote extensive resources to further secure our systems," Nasdaq OMX Chief Financial Officer Adena Friedman and Vince Palmiere, the VP of investor relations, said in a statement. "Cyber attacks against corporations and government occur constantly," they added.

Nasdaq OMX, which runs equity and derivatives exchanges in the United States and Nordic European countries, did not give details on the hackers or on what they were doing. "It's nearly impossible to determine where it comes from, but the authorities are tracking it," the report said.

According to the report, Nasdaq OMX said it had been waiting until at the earliest February 14 before telling its customers, based on a U.S. Justice Department request, to facilitate the investigation.