The Federal Communications Commission faced sharp questioning from a trio of appellate court judges Friday as the agency sought to fend off accusations it had violated the law in repealing the government’s net neutrality rules for Internet providers last year.

In a sixth-floor courtroom in downtown Washington, Tom Johnson, the FCC’s general counsel, tussled with the three-judge panel over the technological nuts and bolts of Internet service, as well as whether public safety officials could be negatively affected by the FCC’s revised rules and wherther states can rightfully establish their own, tougher net neutrality rules.

The legal showdown is the latest in a string of court battles over the policies that will govern the future of the Internet. It underscores the tense relationship between Silicon Valley entrepreneurs and the powerful broadband providers they rely on to reach consumers. State and local officials, consumer groups and tech companies are asking the U.S. Court of Appeals for the D.C. Circuit to countermand the FCC, which argued the rules it repealed harmed Internet providers by raising their costs of doing business and discouraging them from upgrading their networks.

Although Judge Patricia Millett began the day by trying to poke holes in the arguments of the FCC’s opponents, she reserved her most incisive questioning for Johnson’s testimony.

Millett repeatedly challenged the FCC’s assertion that high-speed Internet service is fundamentally different from telephone service and thus deserves to be regulated with a lighter touch. That argument, which draws on a pivotal Supreme Court precedent from 2005, is at the core of the FCC’s defense.

Whether consumers log on to their pharmacy’s website to renew their prescriptions or “push a bunch of buttons” on the telephone, the end result is the same, Millett said.

Johnson replied that “voice service does not provide the same dynamic experience … as accessing the Internet.” He argued that the difference was one of both “kind and degree,” and that Internet service comes with various computer processing functions that are not inherent to telephone service, such as translating URLs into IP addresses and vice versa. Millett responded that assistive services for the blind and hard of hearing are still considered telecommunications services despite featuring similar interpretive functions.

Millett and a colleague, Judge Robert Wilkins, also zeroed in on complaints by local officials that the FCC abandoned its duty to consider first responders when revising its rules — a violation of the agency’s congressional mandate, according to critics. An attorney for Santa Clara County, Calif., argued in court that allowing Internet providers to charge websites additional fees for speedier delivery of their content could disrupt the ability of public health officials to disseminate information in a crisis, and that the FCC failed to account for it.

Wilkins suggested that the FCC could have written a different net neutrality rule that permitted so-called “paid prioritization” or “Internet fast lanes” — satisfying Internet providers — while preserving a ban on that conduct when it came to police and fire officials. Johnson said he was not sure whether such a rule could pass judicial muster.

Judge Stephen Williams at several points leaped in to supply Johnson with credible arguments, reserving much of his critical questioning for opponents of the FCC. Williams pointed out, in one debate over the source of the FCC’s powers, that the agency could still rely on different sources of authority even if the ones used to justify the net neutrality repeal were ultimately rejected by the court.

“It’s clear that two of the judges had very serious problems with the commission’s defense,” said Andrew Schwartzman, a lecturer in law at Georgetown University who attended the arguments.

Broadband industry groups offered a different view.

“Based on today’s arguments, we remain confident that the D.C. Circuit will uphold [the FCC order],” said USTelecom, a trade association representing Internet providers. “[It] is consistent with Supreme Court precedent. Only two years ago, the court said it would leave the decision of Internet service classification to the expert agency — a precedent we believe it should follow.”

Many audience members said the FCC’s opponents appeared to struggle in the initial hours of debate but seemed to strengthen as the hearing wore on. Several high-profile officials, including Sen. Edward J. Markey (D-Mass.); Makan Delrahim, the assistant attorney general for antitrust; and FCC Commissioner Jessica Rosenworcel were in attendance. Also spotted were Tom Wheeler — the former FCC chairman who proposed the net neutrality rules that were later repealed — a senior aide to FCC Chairman Ajit Pai and various attorneys for AT&T, Verizon and other Internet providers.

The aide, Nick Degani, declined to comment for this report.

Alan Davidson, Mozilla’s vice president of global policy, said the maker of the Firefox Web browser is “intensely proud to be here today as part of a broad coalition fighting for an open and free Internet for consumers.” He added, “We argued today that [the FCC] can’t renounce these essential consumer protections on a whim."

Without the protection of the net neutrality rules, Mozilla has argued, Internet providers will be unconstrained in their ability to steer customers toward proprietary or partner services, reshaping the Internet to their own commercial benefit and to the detriment of ordinary users.

With Friday’s oral arguments, the FCC has become the latest Trump-era federal agency to find itself in court after a slew of deregulatory decisions.

The court fight marks the second time in four years the FCC has faced judicial review over its Internet policies. In 2015, the positions were reversed, when Obama-era telecom regulators successfully defended the net neutrality rules from a lawsuit by Internet providers.

Having failed to defeat the rules in court, opponents of the regulations sought to change them from within. FCC officials argue that Pai acted well within the agency’s authority when he held a vote in 2017 to reverse how the FCC oversees Internet providers. Agency officials say they are simply returning to a time when Internet providers were more lightly policed — citing a landmark 2005 Supreme Court ruling that gave the FCC broad discretion to decide how to regulate broadband.

“The U.S. Supreme Court has already affirmed the FCC’s authority to classify broadband as a Title I information service, and we have every reason to believe that the judiciary will uphold the FCC’s decision to return to that regulatory framework,” Matthew Berry, FCC chief of staff, said Thursday in a statement.

Berry said that since the net neutrality rules came off the books last summer, the Internet has continued to thrive without the disastrous consequences activists had predicted. Under the new rules, Internet providers may give preferential treatment to some websites or disfavor others, so long as they disclose the fact — or else they risk an investigation by the Federal Trade Commission. Industry officials have said they are not interested in blocking or slowing down consumers' Internet experiences.

Opponents of the FCC rejected that claim this week, citing research from Northeastern University that suggests some wireless carriers may have slowed down service to Netflix and other video providers. They also said that Internet providers are largely “on good behavior” now because of the ongoing court battle but could easily alter their behavior down the road.

FCC officials say that argument is implausible, pointing out that a future FCC could impose tougher net neutrality rules, or Congress could write legislation cracking down on Internet providers should they go too far.

Critics of the agency argue the FCC rushed to judgment, cherry-picking economic evidence to justify the repeal and forsaking its congressionally mandated mission to serve competition and the public interest. The FCC says it presented a reasoned analysis based on the evidence it had at its disposal and that it has legal precedent on its side.

Which team is on the right side of the law will be up to the three judges to decide.