FERGUSON, Mo. — In the end, it seems, it all came down to Officer Darren Wilson himself.

In four hours of vivid testimony before the St. Louis County grand jury in September describing his shooting of Michael Brown, the officer said Mr. Brown, 18, had looked “like a demon” when he first approached him.

The officer described himself as utterly terrified when, he said, Mr. Brown reached into his police vehicle and fought him for his gun. Mr. Brown was so physically overpowering that the officer, who is 6-foot-4, similar to Mr. Brown, said he “felt like a 5-year-old holding on to Hulk Hogan.”

The officer’s testimony, along with thousands of pages of grand jury documents, including contradictory witness accounts, appeared to have helped convince some of the jurors that the officer had committed no crime when he killed Mr. Brown.

The St. Louis County prosecutor, Robert P. McCulloch, said he had released the documents to show people how thorough the grand jury inquiry had been and to convince the public that justice had been done.