“This is not that complicated a deal from an antitrust or a regulatory perspective,” Mr. Cohen said when the deal was announced. “It presents a lot fewer issues than the NBCUniversal transaction did.”

That position did not convince skeptical regulators — the Justice Department, which had to determine whether a deal would harm competition, and the Federal Communications Commission, which would decide whether it was in the public interest. On Monday, Mr. Roberts met with Tom Wheeler, chairman of the F.C.C., according to a government filing made by Comcast. During the meeting, Mr. Roberts made one last pitch as to why the transaction was in the public interest and promised that Comcast would deliver “best-in-class” service to its new customers. He said that the company would live up to the commitments it made, and as evidence pointed to its earlier deals.

Image Brian L. Roberts is the chief of the Comcast Corporation. Credit... Susan Walsh/Associated Press

Then on Wednesday, Comcast officials met with officials from both the F.C.C. and the Justice Department and met with stiff resistance from both agencies.

One of the biggest concerns of the Justice Department, according to two former antitrust lawyers for the department who were briefed on the agency’s concerns, was that Comcast, with such a large footprint in broadband and cable television operations, could use its clout to place restraints on television networks. For example, it could pressure networks not sell their content through stand-alone Internet streaming services, like those offered by HBO and CBS, that let consumers watch programming without paying for a traditional cable subscription.

The death knell for the deal came on Wednesday when Jonathan Sallet, general counsel of the F.C.C., met with staff members. He told them that his office was going to recommend that the transaction be referred to a hearing before an administrative law judge, multiple people involved in the meeting said.

That hearing process is long and drawn-out, and was essentially a way of saying the deal would be blocked, said Robert M. McDowell, who until 2013 served on the commission and is now in private practice. He said that in his 25 years of observing actions by the F.C.C., he could not recall a transaction being approved after such a referral took place. In 2011, the F.C.C. formally proposed the same path when considering the AT&T and T-Mobile bid to merge. Within a week the two companies had withdrawn their application.