A hacker attack Thursday shut down the fast-growing messaging service Twitter, and Facebook also said it was looking into possible site problems.

Twitter said in its status blog Thursday that it was "defending against a denial-of-service attack," in which hackers command scores of computers to a single site at the same time, preventing legitimate traffic from getting through.

The Twitter outage began at about 9 a.m. EDT, said Ken Godskind, chief strategy officer at Web performance monitoring company AlertSite.

The site still had lingering access problems midday, though both Twitter and Facebook seemed to be functioning at least intermittently.

Incidentally, Facebook also seemed to be experiencing problems. Company spokeswoman Brandee Barker said the company was looking into it and would have an update "as soon as possible."

Technology business analyst Shelly Palmer told AP Radio that denial-of-service attacks are a reality of the information age.

"People tend to want to take sites that are very public and go after them," said Palmer, managing director of Advanced Media Ventures Group. "In fact you'd be surprised how many sites for major companies are really attacked on a daily basis. This is a crime, it's a real crime and it should be treated that way."

Earlier this week, Gawker Media, which owns the eponymous media commentary blog and other sites, was also attacked. In a blog post, Gawker said Tuesday it was attacked by "dastardly hackers," leading to server problems that caused network-wide outages Sunday and Monday. It was not immediately clear whether those attacks were related to Twitter's.