Weeks before the whistleblower's complaint became public, the CIA's top lawyer made what she considered to be a criminal referral to the Justice Department about the whistleblower's allegations that President Donald Trump abused his office in pressuring the Ukrainian president, U.S. officials familiar with the matter tell NBC News.
The move by the CIA's general counsel, Trump appointee Courtney Simmons Elwood, meant she and other senior officials had concluded a potential crime had been committed, raising more questions about why the Justice Department later declined to open an investigation.
Elwood, the CIA's general counsel, first learned about the matter because the complainant, a CIA officer, passed his concerns about the president on to her through a colleague. On Aug. 14, she participated in a conference call with the top national security lawyer at the White House and the chief of the Justice Department's National Security Division. [...]
Justice Department officials now say they didn't consider the phone conversation a formal criminal referral because it was not in written form. A separate criminal referral came later from the Office of the Director of National Intelligence, which was based solely on the whistleblower's official written complaint.
It's unclear from that if the DOJ is prohibited from considering non-written complaints altogether, or if it is just within their latitude to do so. So, what is the legal status of non-written complaints DOJ hears about? (You can narrow the answer to criminal referrals, if that makes a difference.)
According to Wikipedia
These [criminal] referrals may not require formal documentation, but may include a case report.
That fact is supposedly based on Rabin, Robert L. (June 1972). "Agency Criminal Referrals in the Federal System: An Empirical Study of Prosecutorial Discretion". Stanford Law Review. 24 (6): 1036–1091. doi:10.2307/1227884. JSTOR 1227884.
The source cited is nearly 50 years old. So I think the question is still worth asking if nothing really changed in the meantime.
For completeness, I should mention that the DOJ dismissed the later & written complaint too, but they provided a different rationale for that action, namely
A senior DOJ lawyer who briefed reporters said the department had no basis on which to open a criminal investigation because Trump's request of Ukrainian President Volodymyr Zelenskiy to investigate a case involving his political opponent couldn't amount to a quantifiable "thing of value" under campaign finance law.
However that's not the topic of my question.