Top officials at the Democratic National Committee criticized and mocked Senator Bernie Sanders of Vermont during the primary campaign, even though the organization publicly insisted that it was neutral in the race, according to committee emails made public on Friday by WikiLeaks.

WikiLeaks posted almost 20,000 emails sent or received by a handful of top committee officials and provided an online tool to search through them. While WikiLeaks did not reveal the source of the leak, the committee said last month that Russian hackers had penetrated its computer system.

Among the emails released on Friday were several embarrassing messages that suggest the committee’s chairwoman, Representative Debbie Wasserman Schultz of Florida, and other officials favored Hillary Clinton over Mr. Sanders — a claim the senator made repeatedly during the primaries.

In one of the emails, dated May 21, Mark Paustenbach, a committee communications official, wrote to a colleague about the possibility of urging reporters to write that Mr. Sanders’s campaign was “a mess” after a glitch on the committee’s servers gave it access to Clinton voter data.