It seemed like a match made in media heaven. AT&T is a telecommunications giant whose reach stretches to millions of people all over the country, and Time Warner, the owner of CNN, HBO and Warner Bros., has content galore. Together, the two companies would create a colossus straddling the worlds of internet access, news and entertainment.

Until last week, AT&T’s pending $85.4 billion acquisition of Time Warner seemed destined to close by the end of the year. On Wednesday, however, tensions between the Justice Department and executives at the two companies spilled out into the open.

Now it seems possible that the Justice Department and AT&T will end up battling each other in court. The ongoing negotiations have also demonstrated how the Trump administration may regulate big-ticket mergers and acquisitions, representing the first major test for the government’s antitrust strategy.

A central component of the dispute, according to people from both companies and the Justice Department, is CNN — the network that Mr. Trump has frequently attacked as a purveyor of “fake news.”