NEW YORK -- Comcast Corp.'s interference with Internet traffic has prompted a federal investigation and is at the center of calls for "Net Neutrality," or equal treatment of traffic by Internet service providers.

However, another U.S. cable company appears to be doing the same thing without drawing scrutiny.

A study released yesterday found conclusive signs that file-sharing attempts by subscribers of Cox Communications were blocked, along with customers at Comcast and Singapore's StarHub.

Of the 788 Comcast subscribers who participated in the study, 62 percent had their connections blocked. At Cox, 54 percent of subscribers examined were blocked, according to Krishna Gummadi at the Max Planck Institute for Software Systems in Saarbruecken, Germany. The institute examined the network connections of 8,175 Internet subscribers around the world.

"This research proves beyond a shadow of a doubt that consumers, Congress and the FCC must urgently pursue the complaints against network providers," said Ben Scott, policy director of Free Press, one of the groups that urged the Federal Communications Commission to fine Comcast.

FCC spokesman Rob Kenny said the agency would review further complaints about Internet blocking "expeditiously."

At least since 2006, Cox's subscriber agreement has noted that the company engages in "protocol filtering," which means it treats different types of Internet traffic, like Web surfing, e-mail and file-sharing, differently.

Cox said in a written statement yesterday that it takes such steps "to ensure the best possible online experience for our customers." But Cox denied that protocol filtering amounts to discrimination of any specific services.

The blocking observed by Gummadi's group occurs when a subscriber has downloaded a file using the BitTorrent file-sharing program and then tries to upload it over the Internet.

The primary victims are the other Internet subscribers, who will not be able to download a file if a complete version is not available from someone else's computer.

File-sharing programs like BitTorrent, which let people exchange documents, songs, movies and other content, can be heavy users of Internet bandwidth.

Comcast, based in Philadelphia, is the United States' second-largest Internet service provider, with about 14.1 million subscribers.

Atlanta-based Cox Communications is the fourth-largest, with 3.8 million. It serves several areas of Virginia, including Hampton Roads, Roanoke, Fredericksburg and Northern Virginia.

StarHub is Singapore's dominant cable company. It did not reply to an e-mail to its press office.