[Tomisimo]

Good question Gatito.

Use of the imperfect subjunctive as opposed to the the present subjunctive

If the verb in the main clause is in one of the past tenses or the conditional tense (specifically the imperfect indicative, preterit, pluperfect indicative, or conditional) then the imperfect subjunctive is used instead of the present subjunctive.

In this case the verb in the main clause is fue suspendido, requiring the imperfect subjunctive.

The above it true, provided that there is one of the elements that triggers the subjunctive in the sentence.

Conditions that trigger the subjunctive

Perhaps your question in this case is the fact that a gunshot was fired, and that being a concrete happening, why is the subjunctive needed?

The dichotomy between something concrete and something likely to happen is not the only reason the subjunctive is needed.

So what triggers the subjunctive in this case?

Most (all?) of the time-related conjunctions trigger the use of the subjunctive after them. For example: después (de) que, antes (de) que, luego (de) que, cuando, hasta que, mientras etc. You should use the subjunctive after all of these.

EDIT: When I started my post, no one had answered Gatito yet-- You guys sure are fast!