In the chaotic hours after an assassin’s gunshot rang out at Utah Valley University last Wednesday, Tyler Robinson texted his roommate and romantic partner back home 250 miles away and said, “Drop what you’re doing, look under my keyboard.”

There in their nondescript apartment in the fast-growing conservative Utah city of St. George lay a note from Mr. Robinson saying, “I had the opportunity to take out Charlie Kirk, and I’m going to take it.”

It would take 33 hours and a frantic manhunt that roped in top-ranking Trump administration officials before Mr. Robinson, 22, was finally apprehended. In the end, it was Mr. Robinson’s own mother who recognized her eldest son’s image on the news and began a painstaking series of phone calls that ended with him in custody.

Criminal charges filed on Tuesday portray Mr. Robinson as a left-leaning assassin with pro-L.G.B.T.Q. views who spent a week planning a murder that has inflamed America’s political hatreds and led to vows of retribution from the highest echelons of government and even talk of civil war.

But court papers and interviews reveal the wrenching realizations that unfolded in private, as Mr. Robinson’s partner and parents in the red-rock deserts of southern Utah realized that the onetime straight-A student and scholarship winner appeared to be the black-clad figure being hunted by law enforcement.

“You weren’t the one who did it right????” Mr. Robinson’s partner wrote to him last Wednesday.

“I am,” Mr. Robinson replied. “I’m sorry.”