Computers belonging to the US government, contractors and companies in the transportation industry were hit by a targeted computer attack in July that yielded password information for hundreds of internet and intranet websites, a computer security vendor said Tuesday.

The malicious code behind the attacks was first detected by computer intrusion prevention vendor Prevx on July 5. In total, the criminals were able to cull 200Mbs information from about 500 computers, including password and login data, Prevx said.

All but one of the computers was located in the US, and many of the computers that were infected were associated with air transportation, according to Mel Morris, CEO of Prevx. “This was a well-coordinated attack by someone wanting to get information on the related sites.”

PCs at the US Department of Transportation (DOT), American Airlines and DOT contractor Booz Allen Hamilton all were compromised, Morris said.

If DOT systems were compromised, the department is unaware of the situation. “We haven't found any record of a problem,” a DOT spokeswoman said Tuesday.

Prevx obtained a sample of the Trojan used in the attacks on Friday of last week. By Saturday, they had tracked down a website, hosted by Yahoo, where information from the compromised PCs was dropped. Because Prevx found a “massive amount” of job applications on the website used by the criminals, the company believes that victims may have downloaded the malicious software while thinking they were applying for a job.

That website, which had probably been hacked by the criminals behind the scam, was shut down on Tuesday, Morris said. It hosted about 200Mbs of password information, and log-files on the server showed the Internet Protocol (IP) addresses of the infected PCs.

Victims of the Trojan, called Win32.PSWSteal.Gen, are told that their hard drive has been encrypted and that they will need to pay $300 (£146) to for a decryption tool.

Morris believes this extortion threat is actually a ruse designed to conceal the attacker’s true motivation: data gathering. “We think the ransomware is to point anyone investigating this down the wrong alley,” he said.

Morris said he is troubled that such a high percentage of the infected PCs are related to the transportation sector, which has been the target of terrorist attacks.

“When we reverse-engineered the IP addresses of those computers, we couldn't believe that this was a daisy chain that led to government-associated sites and to other defence contractors, and to American Airlines,” Morris said. “This was a very highly targeted attack.”

Criminals have increasingly turned to this type of targeted attack in order to collect and eventually sell data from corporations and government agencies, said Marcus Sachs, deputy director of research group SRI International's Computer Science Laboratory.

But while Prevx’s list of compromised organisations is “kind of scary”’ it’s still hard to characterise what was motivating the attack, Sachs said. “I wouldn't necessarily associate it with terrorists,” he said.