PLUM, Pa. -- People tracking cyber hackers said it's becoming more and more difficult to prevent intruders from getting into computer systems.

One local school district has no idea who is hacking their computer network -- or why.

Someone hacked into the Plum Borough School District's Web site over the weekend.

The incident marks the second time since March that the site has been hacked.

"It has been hacked before. We just take the information down. I think whenever you have a Web site, you're vulnerable," said Plum Schools Information Officer Dawn Check.

Check said she was alerted by a teacher that someone hacked into their Web site.

"They hacked into just this section and put up their information," Check said.

The hacker posted several messages calling himself No. 1 Raider Hacker.

Another message displayed a Web site -- cyberraider.com.

The School of Information Sciences at the University of Pittsburgh said hackers fit one of three profiles.

The first group hacks into systems just to show that they're clever enough to get in.

Another group pops into systems to steal information.

And the most sinister group tries to collapse its target system by launching viruses.

In Plum's case, their hacker got in just for kicks.

"None of our sensitive information, student records, were compromised or hacked into. Not this time," Check said.

"Our systems are connected to each other. They're exposed to open Internet environment so there are vulnerabilities," said James Joshi, a professor at the University of Pittsburgh.

That's why every system should be protected with a firewall, and hardline policies that govern access into Web sites should be established, Joshi said.

"Making sure you have proper authentication and access control, mechanisms in place," Joshi said.