The LAPD's Hollywood Division received a statement from the alleged victim Tuesday, the source said.
Investigators will be giving the report, once completed, to the LAPD's Robbery/Homicide Division, which typically handles high-profile cases, according to the law enforcement source.
Westwick, meanwhile, has publicly denied the allegation on Twitter.
"I do not know this woman," Westwick wrote. "I have never forced myself in any manner, on any woman. I certainly have never committed rape."
The woman, Kristina Cohen, detailed her allegation in a public Facebook post Monday.
"I hope my coming forward will help others to know that they are not alone, that they are not to blame, and it is not their fault. Just as the other women and men coming forward have helped me to realize the same," she wrote.
WARNING: The below Facebook post contains graphic language.