The back-and-forth over AT&T’s proposed merger with T-Mobile continued Monday, as opponents filed yet another round of documents at the Federal Communications Commission, arguing that the deal is everything from unnecessary to illegal.

Monday was the final deadline for opponents to file comments with the FCC. Groups had until the end of May to file official petitions to deny with the FCC. AT&T and its supporters responded to those petitions on June 10.

In its highly detailed filing, rival wireless carrier Sprint argued that AT&T could expand its network capacity without acquiring T-Mobile and for a fraction of the merger’s $39 billion price tag.

“AT&T could increase its network capacity by more than 600 percent by 2015 without subjecting the country to the anti-competitive and anti-consumer harms associated with its proposed takeover of T-Mobile,” a Sprint spokesperson said in a statement.

The Computer & Communications Industry Association said AT&T has “failed to refute claims that the merger will result in unprecedented, dangerous market concentration both horizontal and vertical.”

The advocacy group Public Knowledge went as far as to call the merger illegal, and pressed the FCC to block the transaction.

The group also said AT&T has not been honest with lawmakers, promising Congress it would not take Universal Service Fund subsidies, then backtracking in documents filed with the FCC.

“Given the speed with which AT&T’s lawyers rewrote a public pledge under oath before Congress to accept a merger condition to use only ‘private capital’ for its much touted LTE deployment into a ‘voluntary commitment’ while simultaneously asserting its right to apply for the maximum USF subsidy allowable, the FCC must ask how long it will take before AT&T rewrites its other ‘voluntary commitments’ on deployment, network investment, and job creation,” Public Knowledge’s filing states.

Other groups that filed in opposition included Free Press and the Greenlining Institute.

Supporters of the merger fired back in filings and statements of their own.

“By expanding the reach of high speed mobile broadband service to more than 97% of the country, this transaction will help pave the way for more broadband usage, significant economic opportunities and astounding mobile innovation,” said Jonathan Spalter, chairman of Mobile Future, which filed comments with the FCC in favor of the deal.

And Communications Workers of America reiterated its support, saying that a combined AT&T/T-Mobile will benefit workers.