Hackers targeted the computer systems of presidential candidate Donald Trump and Republican Party organizations as well as Democratic Party networks, sources familiar with investigations into the attacks said.

At least one Trump staff member's email account was infected with malware in 2015 and sent malicious emails to colleagues, according to one insider for the Republican candidate's campaign and an outside security expert. It was unclear whether or not the hackers actually gained access to campaign computers.

In the past month, U.S. security officials have said that starting last year, hackers infiltrated computers of the Democratic National Committee (DNC), the presidential campaign of Hillary Clinton and her party's congressional fundraising committee.

U.S. officials said they have concluded that Russia or its proxies were responsible, leading to calls by some Democrats and cyber security officials for the Obama administration to blame Russia publicly. Kremlin officials have dismissed the allegations as absurd, but there is anxiety in Washington over the possibility that a foreign power might be using hacked information to meddle in the Nov. 8 U.S. election.

The Trump campaign has hired security firm CrowdStrike, which also is assisting the Democratic National Committee, according to one person briefed on the matter. The company declined to comment.

A different outside security firm was hired to examine software the Trump and Clinton campaigns use to manage mailings, electronic outreach and other campaign efforts, another person who was briefed on the issue said.

A spokeswoman for Trump's campaign declined to comment. A spokesman for the Republican National Committee could not immediately be reached for comment.

The tools and techniques used to hack Republican targets resemble those employed in attacks on Democratic Party organizations, including the DNC and Clinton's campaign organization, two sources said. That has led U.S. officials to reach a preliminary assessment that Russia's military and civilian intelligence agencies or their proxies have targeted both political parties.

Attempts to hack into Republican political organizations over an extended period were reported in intelligence bulletins circulated by U.S. agencies, four sources said. They did not disclose the identities of the organizations.

Two U.S. security officials said the FBI and the Department of Homeland Security have offered assistance to both political parties in identifying possible intrusions and upgrading their defenses against what one of the officials called "constantly evolving threats."