The fact that Patrick Maloney was a policeman has considerable significance. It was because of his profession that his wife, Mary, knew exactly what to do in order to avoid suspicion resting on her after she had killed him.
Mary obviously knew all the requirements for guilt to be proven in the committal of a crime since she and Patrick must have spoken about his work at length. She would know exactly what kind of evidence was needed and what detectives would be looking for. It was for this reason that she carefully constructed a seemingly perfect alibi: she went to the store to buy groceries and acted as if Patrick were still alive. She made sure that the grocer would think that Patrick had arrived home from work and was expecting her back with the goods to use for their supper. The text makes this quite clear:
"Hullo Sam," she said brightly, smiling at the man behind the counter.
"Why, good evening, Mrs. Maloney. How're you?"
"I want some potatoes please, Sam. Yes, and I think a can of peas."
The man turned and reached up behind him on the shelf for the peas.
"Patrick's decided he's tired and doesn't want to eat out tonight," she told him. "We usually go out Thursdays, you know, and now he's caught me without any vegetables in the house.
She continues to speak in the present tense, further emphasising the fact that Patrick was alive, waiting for her to return. Once Mary had established an alibi, she made sure that she called the police station and act grief-stricken on having 'discovered' her husband's corpse.
A few minutes later she got up and went to the phone. She know the number of the police station, and when the man at the other end answered, she cried to him, "Quick! Come quick! Patrick's dead!"
"Mrs. Maloney. Mrs. Patrick Maloney."
"You mean Patrick Maloney's dead?"
"I think so," she sobbed. "He's lying on the floor and I think he's dead."
Mary knew that the officers would arrive quickly since Patrick was a colleague. They would conduct a thorough investigation. She realised that they would leave no stone unturned since Patrick was one of their own and they would be determined to find the perpetrator. Furthermore, because of this, they would be very sympathetic since she was now the widow of a fellow officer and they would extend her every possible kindness to spare her further grief.
Mary was very convincing and, to further assert her innocence, asked whether her husband was 'really dead'. The officers responded exactly as she wanted:
... the detectives kept asking her a lot of questions. But they always treated her kindly.
Being a policeman's wife, Mary knew that finding the murder weapon would be uppermost in the investigators' minds. They would search every nook and cranny until they discovered it. As Sergeant Jack Noonan put it:
"It's the old story," he said. "Get the weapon, and you've got the man."
It is for this reason that Mary found a very creative way to get rid of the murder weapon. She had placed the leg of lamb in the oven and Sergeant Noonan later told her that it was still there and enquired whether he should switch off the oven. Mary then asked him if they would do her a small favour:
"Well," she said. "Here you all are, and good friends of dear Patrick's too, and helping to catch the man who killed him. You must be terrible hungry by now because it's long past your suppertime, and I know Patrick would never forgive me, God bless his soul, if I allowed you to remain in his house without offering you decent hospitality. Why don't you eat up that lamb that's in the oven. It'll be cooked just right by now."
At first the officers refused but after she insisted, and since they were famished, finally gave in to her request. The policemen then tucked into the evidence, destroying it - piece by succulent piece.
And so, Mary Maloney had committed the 'perfect' crime.