Consider the following sentence:
The main culprit together with his associates was/were arrested.
Which should be used, was or were?
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It should have commas, and be:
The main culprit, together with his associates, was arrested.
The "was" is seen to be applying to the culprit, not the associates, and so is singular.
Also, "together" would be better replaced with "along". I think the whole sentence would be better written as
The main culprit was arrested, along with his associates.