"unknown perpetrator" is correct and usable in written English.
You can use it when you want to refer to someone who has committed an illegal act, but their identity is unknown. For example: The police are still searching for the unknown perpetrator of the bank robbery.
In the days following the bombings at the Boston Marathon, speculation online regarding the identity and motive of the unknown perpetrator or perpetrators was rampant.
Peculiarly, Duda's case was taken to trial with an "unknown perpetrator," which, as you can tell, means absolutely nothing.
"An unknown perpetrator or perpetrators are blackmailing this state, threatening to spread the Ebola virus," Zdenek Laube, the country's deputy police chief, told reporters, according to AFP.
It's one of the reasons why we're reluctant to call the distinctly Anglo-Saxon Martin Bryant (religion unknown), perpetrator of the Port Arthur massacre, a terrorist; it's one of the reasons that far-right Scandinavian Christian Anders Behring Breivik, perpetrator of the 2011 Norwegian attacks, was rarely referred to in the press as a terrorist, despite being literally convicted of terrorism.
Exposure from known perpetrator was more strongly associated with PPD than exposure from an unknown perpetrator.
For example, we do not know if the prevalence of sexual violence and associations investigated differ significantly for a known compared to unknown perpetrator.
By applying the word to a [Boston Marathon] bombing with initially unknown perpetrators, and largely declining to use it in connection with a massacre allegedly perpetrated by a white supremacist hoping to spark a race war, media failed that test.
Nevertheless, the few studies of criminal investigation that have been conducted stress the limited contribution of such scientific methods to the identification of unknown perpetrators.
Initially, the system was used much less to identify unknown perpetrators than to allow investigators to determine whether the suspects they arrested had been involved in previous crimes.
She was born in Soweto, in a family of 10 children, four of whom died young, one of them stabbed to death by unknown perpetrators when he was 21.
In its 2007 human-rights assessment, the US State Department wrote: "Credible reports cited unlawful killings by government agents, assassinations by unknown perpetrators, politically motivated killings and child soldier recruitment by paramilitary forces associated with the government".The government's response to this has been depressing.
