A hacker who American intelligence officials believe has ties to the Russian government made public on Tuesday a second batch of documents suspected of having been stolen from the Democratic National Committee’s computer system, leaving the organization rushing to contain damage or embarrassment less than two months before the presidential election.

The internal documents, which party officials were verifying for authenticity, were distributed at a cybersecurity conference in London.

The release was apparently the work of Guccifer 2.0, a self-identified Romanian hacker who claimed responsibility for publishing a trove of the committee’s documents before the Democratic convention in July.

Donna Brazile, the interim chairwoman of the Democratic National Committee, blamed Russian espionage for the hack and said that foreign agents were trying to help Donald J. Trump, the Republican presidential nominee, be elected.