Also see co-perpetrator if it fits the context.
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(law) One of two or more people accused of the same offence
https://en.wiktionary.org/wiki/coaccused
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one of two or more people or organizations accused of the same crime
https://www.macmillandictionary.com/us/dictionary/american/c...
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Evidence that the co-perpetrator who assisted or facilitated the commission of the crime had the same intent than the person who physically perpetrated the crime.

https://www.casematrixnetwork.org/cmn-knowledge-hub/proof-di...
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A co-perpetrator is someone who pursues a criminal endeavor with at least one other confederate, sharing joint control over the operation.
https://scholarship.law.cornell.edu/cgi/viewcontent.cgi?arti...

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In law enforcement jargon, a suspect is a known person accused or suspected of committing a crime. Police and reporters in the United States often use the word suspect as a jargon when referring to the perpetrator of the offense (perp in dated US slang). However, in official definition, the perpetrator is the robber, assailant, counterfeiter, etc.—the person who committed the crime. The distinction between suspect and perpetrator recognizes that the suspect is not known to have committed the offense, while the perpetrator—who may not yet have been suspected of the crime, and is thus not necessarily a suspect—is the one who did. The suspect may be a different person from the perpetrator, or there may have been no actual crime, which would mean there is no perpetrator.
https://en.wikipedia.org/wiki/Suspect

Synonyms

* abettor, accessory, assistant, associate, confederate, coadjutor, ally, promoter; see abettor.

a fellow suspect / a co-suspect

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Note added at 55 mins (2019-09-11 12:19:42 GMT)
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a fellow suspect
https://www.google.com/search?client=firefox-b-d&q="fellow s...
a co-suspect
https://www.google.com/search?client=firefox-b-d&sxsrf=ACYBG...

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Note added at 20 hrs (2019-09-12 07:52:33 GMT)
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United States Court of Appeals, Seventh Circuit.
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finding an interrogator's lie that a fellow suspect had confessed insufficient to make an otherwise voluntary confession inadmissible
https://caselaw.findlaw.com/us-7th-circuit/1882179.html

District Court of Appeal, First District, Division 2, California.
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In Escobedo, defendant said to a fellow suspect, in the presence of police, “‘I didn't shoot Manuel, you did it.”’ In the instant case, appellant said to Officer Klapp in effect, ‘I didn't take the paint from the shed, Davidson and Robertson did.’
https://caselaw.findlaw.com/ca-court-of-appeal/1818604.html

United States Court of Appeals,First Circuit.
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In such a circumstance where the referenced relative is both a family member and a co-suspect, probable cause for holding that individual helps to place the officers' statements in context.
https://caselaw.findlaw.com/us-1st-circuit/1694809.html

United States Supreme Court
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The fact, then, is that the language of the accused, which was offered in evidence as a confession, was made use of by him as a reply to the statement of the detective that Bram's co-suspect had charged him with the crime...
... one who is held in custody under suspicion of having committed a crime would not be impelled to say something when informed by one in authority that a co-suspect had declared that he had seen the person to whom the officer was addressing himself commit the offense...
... To communicate to a person suspected of the commission of crime the fact that his co-suspect has stated that he has seen him commit the offense...
https://caselaw.findlaw.com/us-supreme-court/168/532.html