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How can helping another person commit a crime create criminal responsibility?

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Collecting fingerprints — crime Police officer collecting fingerprints.

Criminal responsibility for helping another person commit a crime arises when an individual knowingly and voluntarily aids, encourages, or incites the commission of an offense. Under the law, such a person is generally considered an accomplice, a category that includes both accessories and abettors.123

Abettors (Principals in the second degree): These individuals are present—either actually or constructively—during the commission of the crime and provide assistance, encouragement, or incitement.12
Accessories before the fact: These individuals assist, procure, or counsel the offender before the crime occurs, such as by providing information or tools.12

Modern statutes have largely abolished the traditional common-law distinctions between these roles, often treating all accomplices as principals who are equally liable for the crime. Furthermore, individuals may face criminal responsibility through conspiracy, which involves an agreement between two or more people to commit an unlawful act.234

It is important to note that simply selling a legal item, such as a rifle, does not automatically make one an accessory to a crime committed with that item; evidence of an intent to facilitate the specific offense is typically required.2