At least three months before the revelation that former Representative Anthony D. Weiner was sending lewd messages and photos to women online, a small group of self-described conservatives was monitoring his exchanges with women on Twitter. Now there is evidence that one or more people created two false identities on Twitter in order to collect information to use against him.

A Twitter user employing a fake name posed as a 16-year-old California high school girl in May and tried to get Mr. Weiner to be her prom date, according to people with knowledge of the communications and a review of documents. The person behind another Twitter account created under a fake name claimed to be her classmate and offered to provide the group with incriminating evidence about Mr. Weiner.

Mr. Weiner, who resigned on Thursday after admitting he had sent explicit photos and messages to multiple women on social media sites, had already been the subject of intense focus on Twitter by the conservative group, which calls itself the #bornfreecrew.

One Twitter user the group observed seeking to interact with Mr. Weiner was called “Nikki Reid.” She started an online campaign to get Mr. Weiner to be her prom date at Hollywood High School in May, using the account @starchild111. Within days after Mr. Weiner started following her, a Twitter user, also using a fake name, Marianela Alicea, and pretending to be Nikki Reid’s classmate, contacted a member of the #bornfreecrew and said she had information about Mr. Weiner, but never provided any.