Criminal Procedure of Homicide Questions

I’m trying to study for my Law course and I need some help to understand this question.

Hunters discovered the body of a dead woman floating in an abandoned gravel pit pond. The county coroner ruled that the woman’s death was a homicide. Her ex-husband, Fred Johnson, who had read accounts in the local paper, was considered a suspect. He called the police department to inquire if the body in the pond might be the body of his ex-wife. Police asked him to come to headquarters, purportedly to identify personal items they thought belonged to dead woman. In reality, police wanted to interrogate Fred Johnson about the murder of his ex-wife.

Fred Johnson drove his truck over to police headquarters and immediately identified the personal property as belonging to his former spouse. Fred Johnson stayed at the police headquarters for two hours while two police officers questioned him in a small interview room. The interrogation was thorough, but not overly intimidating. The officers openly tape recorded the interrogation. At no time did the police officers inform Fred Johnson of his Miranda rights. The officer repeatedly assured Johnson that he was not under arrest and that he was free to terminate the questioning and leave the police station. Since the police had focused on Johnson as the culprit, they indicated that they were sure that he had killed the former Mrs. Johnson.

In order to place some additional pressure on Fred Johnson, police told him that other officers were conducting a search for evidence at his home and that police were going to search his pickup truck for evidence of a homicide. Police asked various questions that had the goal of getting Fred Johnson to confess to the homicide of his former wife. In the latter part of the interrogation, Johnson indicated that he had indeed killed her and put her body in the gravel pit pond and hoped that her body stayed submerged. As promised, police allowed Fred Johnson to leave the police station and go about his business, although police did seize his pickup truck. Several hours later, police found Johnson and arrested him for the murder of his ex-wife.

Fred Johnson argued that the confession given at the police station should not be admitted against him since his rights under Miranda v. Arizona were violated since he was in custody during questioning.

The Procedure Problem
Should police have offered the Miranda warnings to Fred Johnson under the above circumstances?

Should the trial court rule that his confession had been illegally obtained?

Identify the factual issues and apply the legal concepts dictated by the Miranda warnings and Miranda-based case law.?

Be sure to arrive at a definite conclusion concerning whether the Miranda warnings were necessary to be given to Fred Johnson.?