Adding that the F.B.I. did not comment.

Adding comments from Curbed Network and Instapaper.

Adding comment from DigitalOne.

9:22 p.m. | Updated Adding additional comments from DigitalOne.

The F.B.I. seized Web servers in a raid on a data center early Tuesday, causing several Web sites, including those run by the New York publisher Curbed Network, to go offline.

The raid happened at 1:15 a.m. at a hosting facility in Reston, Va., used by DigitalOne, which is based in Switzerland, the company said. The F.B.I. did not immediately respond to a request for comment on the raid.

In an e-mail to one of its clients on Tuesday afternoon, DigitalOne’s chief executive, Sergej Ostroumow, said: “This problem is caused by the F.B.I., not our company. In the night F.B.I. has taken 3 enclosures with equipment plugged into them, possibly including your server — we cannot check it.”

Mr. Ostroumow said that the F.B.I. was only interested in one of the company’s clients but had taken servers used by “tens of clients.”

He wrote: “After F.B.I.’s unprofessional ‘work’ we can not restart our own servers, that’s why our Web site is offline and support doesn’t work.” The company’s staff had been working to solve the problem for the previous 15 hours, he said.

Mr. Ostroumow said in response to e-mailed questions that it was not clear if the issues would be resolved by Wednesday.

A government official who declined to be named said earlier in the day that the F.B.I. was actively investigating the Lulz Security group and any affiliated hackers. The official said the F.B.I. had teamed up with other agencies in this effort, including the Central Intelligence Agency and cybercrime bureaus in Europe.

Mr. Ostroumow declined to name the client targeted by the F.B.I. and said that he did not know why it had drawn their interest. It was also unclear why the agents took more servers with them than they sought, he said.

The sites of the Curbed Network, including popular blogs covering real estate, restaurants and other topics, were all unavailable Tuesday evening. Lockhart Steele, Curbed’s president, said his team realized that the company’s sites were down at around 3 a.m. and contacted DigitalOne. After initially declining to say what had happened, DigitalOne explained that the F.B.I. had raided the data center, Mr. Steele said.

“Our servers happened to be in with some naughty servers,” he said, adding that his sites were not the target of the raid. Curbed is working to get its sites back online, probably by Wednesday.

The raid also affected a server used by Instapaper, a popular service that saves articles for later reading. Marco Arment, Instapaper’s founder, said he lost contact with a server hosted by DigitalOne early on Tuesday. Instapaper’s Web site is still operating but has slowed somewhat. Mr. Arment said he had not heard from DigitalOne or law enforcement, and had no reason to believe that Instapaper was a target of the raid.

Pinboard, a bookmarking site, was operating on a backup server and some of its features were turned off, a post on its site said.

DigitalOne provided all necessary information to pinpoint the servers for a specific I.P. address, Mr. Ostroumow said. However, the agents took entire server racks, perhaps because they mistakenly thought that “one enclosure is = to one server,” he said in an e-mail.

DigitalOne had no employees on-site when the raid took place. The data center operator, from which DigitalOne leases space, passed along the information about the raid three hours after it started with the name of the agent and a phone number to call.

Before learning of the raid, Mr. Ostroumow, who is in Switzerland with the rest of his team, thought the problem was a technical glitch, he said.

Nick Bilton contributed reporting.