Dozens of Internet and network security experts from the government and industry are working to discover the origins of last week's attack on Lockheed Martin Corp's computer networks, according to a new report.

The investigators are working at Lockheed's 25,000 square-foot security intelligence center outside Washington D.C., Reuters reported. The attacks developed after a security breach at EMC Corp's RSA security division in March.

The analysts are at a center only open to critical personnel, according to the report. The center monitors data transmissions by 126,000 employees and outsiders trying to get access to the system.

U.S. Defense department officials were at the site last week to assess the wider impact, a defense official who requested anonymity told Reuters on Thursday.

UNKNOWN ATTACKERS

Unknown hackers broke into the security networks of Lockheed Martin Corp and several other U.S. contractors, according to a report last week.

The breach occurred when hackers created duplicates to SecurID electronic keys from EMC Corp's RSA security division, a person not authorized to publicly disclose the matter said in the report.

It was not immediately clear what data, if any was stolen.

Hackers learned how to copy security keys with data stolen from RSA during an attack EMC disclosed in March, the source told Reuters.

Several companies, including General Dynamics, Boeing, Northrop Grumman, Raytheon and other defense firms declined to comment.

In March, EMC Corp. admitted that its security division RSA became a victim of an advanced cyber attack.

EMC said measures instituted to protect the RSA and its customers included hardening our IT infrastructure. The company has begun an investigation and is working with authorities, he said.

The attack type is known as an Advanced Persistent Threat(APT)

Customers were informed of the breach and were being provided ways to strengthen the product's implementation