The massive data breach disclosed by the U.S. Office of Personnel Management (OPM) Thursday shows that federal government efforts at shoring up its cybersecurity remain a work in progress.

In a statement Thursday, the OPM said that in April it discovered an intrusion into IT systems that may have compromised personally identifying information on some 4 million current and former federal employees.

The agency is currently working with the FBI and the U.S. Department of Homeland Security to understand the full scope of the breach. The investigation is still ongoing and it is possible that additional PII compromises will be discovered, the agency cautioned.

“Since the intrusion, OPM has instituted additional network security precautions, including: restricting remote access for network administrators and restricting network administration functions remotely,” the statement said.

OPM has also instituted a review of all connections to ensure that only legitimate business connections have access to the Internet. In addition, the agency will deploy anti-malware tools across its entire environment to prevent the deployment or execution of malicious software that presumably may have been left behind by the intruders.

The agency will begin sending out letters next week informing victims that personally identifying information of theirs may have been potentially compromised in the incident. It will offer free credit monitoring and identity protection services for 18 months to all victims of the breach.

No details are officially available so far on what systems were breached, how they were broken into, or how the intrusion was discovered.  But several media reports quoted unnamed US officials as pinning the blame on China for the attack. The target of the theft appears to have been Social Security Numbers and other personally identifying information, but it is not clear if the attackers were state sponsored or a criminal group, the New York Times said.

The OPM is fundamentally responsible for human resource functions and personnel policy management for the entire federal government. It stores PII on virtually everyone applying for or holding a federal position or wanting to be considered as a federal contractor. So the breach potentially affects individuals from across the entire federal government, making it the biggest of its kind ever.

The breach marks the second time in the past year that hackers have breached OPM’s network. Last July, DHS officials confirmed to the New York Times that intruders had broken some of OPM's databases before they were detected and blocked. In that incident, the attackers appear to have been targeting data on tens of thousands of federal employees who had applied for top-secret security clearances with the OPM, the Times reported. For the moment at least, it's unclear if last year’s incident and the one reported this week are connected.

The OPM breach continues a spate of recent intrusions into federal networks over the past few months. In the space of just one week last fall, both the White House and the U.S. State Department reported that hackers had broken into their networks and accessed what turned out to be unclassified portions of their respective networks.

Just a few weeks prior to that, the U.S. Postal Service announced a breach affecting about 800,000 employees and 2.9 million customers, while the National Oceanic and Atmospheric Administration announced one just a couple of days later.

The incidents show that change is slow with federal cybersecurity, says Chris Wysopal, chief technology officer at application security vendor Veracode.

The company’s data shows that government agencies lag all other sectors in terms of number of serious defects in their application and the rate at which they fix those vulnerabilities. Based on Veracode’s analysis, the government has the highest prevalence of easily exploitable vulnerabilities like SQL injection and cross-site scripting, Wysopal says.

“You would expect the government to protect its personnel data as well as the finance, healthcare, or manufacturing industries but it just isn’t the case,” he said in an emailed statement.

Grayson Milbourne, security intelligence director at Webroot, says that the latest breach shows why government approaches to cybersecurity are in need of reform. “Clearly, the government’s approach to cybersecurity needs to be reformed, prioritized and accelerated,” he said in an emailed statement.

 “That the breach might have been carried out by the Chinese does not absolve the OPM of blame. The issue here is the government’s technological failings and what it should be doing to prevent future attacks,” Milbourne said.