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Locast -- a nonprofit startup that grabs over-the-air channels and streams them free over the internet -- said late Thursday that the four giant US broadcasters' "sham litigation" against it is an antitrust conspiracy to drive it out of business and threaten its supporters. Locast's first official answer to the copyright suit goes beyond simply rejecting the companies' accusations of copyright infringement to accuse them of collusion.

And it's bringing Google's YouTube into the fight. Locast said executives at YouTubeTV -- a paid service that streams live TV channels -- met with the Big Four broadcasters suing Locast in April. According to Locast, the YouTube executives were told that if YouTubeTV provided access to Locast, then YouTubeTV would be "punished" by the media giants when YouTube renegotiated the licensing deals allowing its streaming service to carry media giants' cable networks.

Disney's ABC, CBS, Fox and Comcast's NBCUniversal sued Locast in July. (Note: CBS is the parent company of CNET.)

Later Friday, the attorney representing the broadcasters said Locast's filing "only confirms that it has no answer for its industrial-scale violations of the law."

"We trust the courts to see right through this facade and recognize Locast for what it is -- not a public service organization, but a creature of certain pay-TV interests with an entirely commercial agenda," Gerson Zweifach of Williams & Connolly said in a statement.

The Locast service is available in 13 cities, including New York, San Francisco and Los Angeles.

The suit was the first attack against a company that many see as a successor to Aereo, a for-profit streaming service that five years ago offered consumers livestreams of broadcast channels for a monthly subscription. That came at the height of broadcasters' anxiety about cord-cutting and consumers forsaking them for streaming services, and in 2014 the Supreme Court ruled that Aereo was illegal, leading to its demise.

One difference between the cases, however, is Locast's nonprofit status. US copyright law has allowed certain nonprofit institutions to grab over-the-air TV signals and retransmit them to nonpaying viewers, such as a university setting up an antenna that can retransmit to students in its dorms. Aereo, as a for-profit company, used a different, technological loophole when it argued its service was legal.

Other corporate giants had been bolstering Locast before the suit. AT&T, which owns major pay TV distributor DirecTV, donated $500,000 to Locast and planned to add Locast to its DirecTV and U-Verse set-top boxes. Dish, meanwhile, is making Locast a default app suggested on its new AirTV players.

AT&T also recommended that its customers use Locast while CBS was dark on its services earlier this year.

Originally published Sept. 27, 4:09 a.m. PT.

Update, 2:06 a.m. PT: With broadcasters response.