A hacker claiming responsibility for the recent data breach of the Democractic National Committee apparently has posted the stolen files online.

The hacker, who goes by the name Guccifer 2.0, leaked the files on Wednesday following a breach of DNC computers that has been blamed on Russian hackers.

The posted files include a 231-page dossier containing opposition research on presumptive Republican presidential nominee Donald Trump. They also include documents concerning expected Democratic nominee Hillary Clinton’s election strategy, items on U.S. foreign policy, and donor lists.

“Here are just a few docs from many thousands I extracted when hacking into DNC’s network,” the hacker wrote.

Guccifer 2.0 said the rest of the files have been sent to Wikileaks for posting.

The DNC could not immediately be reached for comment. Details of the breach were announced on Tuesday, but the attacks started as far back as last summer.

Two separate hacking groups with links to the Russian government were involved and are considered among the world's best, according to CrowdStrike, the security firm that the DNC hired to stop the attack. One of the groups, called Cozy Bear, is believed to have hacked networks used by the White House and the U.S. State Department last year.

However, the posting by Guccifer 2.0 implied the attack was carried out by one person. On Wednesday, the hacker mocked CrowdStrike.

“I’m very pleased the company appreciated my skills so highly,” he wrote. “But in fact, it was easy, very easy.”

The name Guccifer refers to a Romanian hacker named Marcel Lehel Lazar who breached Clinton’s personal email server in 2013. He has been extradited to the U.S., where he awaits sentencing after pleading guilty to identity theft and unauthorized access to protected computers.

This new hacker, Guccifer 2.0, is warning that more breaches could come.

“Guccifer may have been the first one who penetrated Hillary Clinton’s and other Democrats' mail servers. But he certainly wasn’t the last,” the hacker said.

CrowdStrike still stands by its findings that Russian government hackers were involved. The company is working to verify the documents' authenticity and origin and said the Guccifer 2.0 postings could be part of a disinformation campaign.