Hackers behind the massive Equifax data breach began their attack no later than early March, more than four months before company officials discovered the intrusion, according to a report published Wednesday by the Wall Street Journal.

The first evidence of the hackers' "interaction" with the Equifax network occurred on March 10, according to the report, which cited a confidential note that security firm FireEye sent to some Equifax customers. By then, a critical vulnerability in the Apache Struts Web application framework was already under active exploit on the Internet . Equifax officials have said the Struts flaw was the opening that gave attackers an initial hold in the targeted network

Equifax has said that the breach that exposed sensitive data for as many as 143 million US consumers started on May 13 and lasted until July 30. The company didn't disclose the breach until September 7.

The attackers, according to the WSJ, eventually entered the command "Whoami," giving them the capability to determine the user account they had compromised. It was likely the beginning of months of painstaking hacking as the attackers attempted to escalate their privileges and intrude further into the Equifax network. Sometime between May and late July, the hackers accessed files that contained Equifax credentials and "performed database queries that provided access to documents and sensitive information stored in databases in an Equifax legacy environment," the report said. Eventually, the attackers accessed "numerous database tables in several databases."

The attackers also managed to set up about 30 Web shells that allowed them to remotely enter the same sorts of powerful commands available to high-privilege Equifax administrators. The hidden pages would remain even after the vulnerable Struts applications on the network were patched. As it turned out, Equifax didn't fix the flaw until July 30.

Mandiant, the FireEye unit that Equifax called in to investigate the breach, said it has detected about 35 IP addresses the attackers used to access the company's network. The hackers' identity remains unknown. Mandiant has been unable to attribute the breach to any hacking groups it currently tracks, and the tools, tactics, and procedures used in the hack don't overlap with those seen in previous Mandiant investigations.