The hacking attempts targeted staff at Washington-based SKDKnickerbocker, a campaign strategy and communications firm working with Biden and other prominent Democrats, over the past two months, the sources said.

A person familiar with SKDK’s response to the attempts said the hackers failed to gain access to the firm’s networks. “They are well-defended, so there has been no breach”, the person said.

SKDK Vice Chair Hilary Rosen declined to comment. A Biden spokesman did not respond to a request for comment.

The hacking attempts on SKDK come as US intelligence agencies have raised alarms about possible efforts by foreign governments to interfere in the November presidential election.

Investigations by former special counsel Robert Mueller and the Senate intelligence committee both concluded that affiliates of the Russian government interfered in the 2016 presidential election, and Mueller has warned that Russia was meddling in the current campaign.

It is unclear whether Biden’s campaign was the target or whether the Russians were attempting to gain access to information about other SKDK clients.

SKDK managing director Anita Dunn was a White House communications director during the Barack Obama presidency and serves the Biden campaign as a senior advisor.

Microsoft has identified hackers tied to the Russian government as the likely culprits. The attacks included phishing, a hacking method which seeks to trick users into disclosing passwords, as well as other efforts to infiltrate SKDK’s network.