The chief executive of a data-analytics firm that worked for President Donald Trump’s campaign reached out to WikiLeaks founder Julian Assange to offer help organizing the Hillary Clinton-related emails the website was releasing, according to a person familiar with the effort.

The outreach by the CEO of the firm, which is partly owned by a major Trump donor and has close ties to a Trump adviser, came as Mr. Trump was publicly cheering the leaks of his Democratic rival’s emails and some supporters were seeking to unearth further messages.

In an email sent in August 2016 and recently reviewed by the person, Cambridge Analytica CEO Alexander Nix told other employees at the firm and Rebekah Mercer, a top Republican donor, that he had recently reached out to Mr. Assange to offer help better indexing the messages WikiLeaks was releasing to make them more easily searchable. Those emails included a trove of messages stolen from Mrs. Clinton’s campaign chairman John Podesta’s account and from the Democratic National Committee.

In his email, Mr. Nix said he had not heard from Mr. Assange, the person said.

On Wednesday, Mr. Assange said he had rejected an approach by Mr. Nix, though he didn’t say what had been offered.