Trump aide O'Brien cuts Europe trip short to deal with cyber hack

The sweeping attacks, first reported by Reuters on Sunday, prompted the U.S. Department of Homeland Security and thousands of businesses to scramble to investigate and respond to the hacking campaign that officials suspect was directed by the Russian government.

O'Brien "is holding NSC meetings tonight and tomorrow morning and will convene a high-level interagency meeting this week," National Security Council spokesman John Ullyot said.

A bipartisan group of U.S. senators, including the top Republican and Democratic members of the Senate commerce panel, wrote a letter to the directors of the FBI and the Cybersecurity and Infrastructure Security Agency requesting a report about the extent of the attacks.

Citing people familiar with the matter, Reuters reported on Sunday that hackers backed by a foreign government had been monitoring internal email traffic at the U.S. Treasury Department and the Commerce Department's National Telecommunications and Information Administration, an agency that decides internet and telecommunications policy.

Technology company SolarWinds, which was the key steppingstone used by the hackers, said up to 18,000 of its customers had downloaded a compromised software update that allowed hackers to spy unnoticed on businesses and agencies for almost nine months.

In their letter, the senators asked for, among other things, a list of all federal agencies that are customers of SolarWinds, the categories and quantities of data that were susceptible to hacking, and any confirmed cases of unauthorized access.

The letter was signed by Republican Senator Roger Wicker, chairman of the Commerce Committee; Senator Maria Cantwell, the committee's top Democrat; and four other senators.

O'Brien had visited Israel and France on his trip but canceled stops in Italy, Germany, Switzerland and Britain. A source familiar with O'Brien's plans said he was now scheduled to return to Europe on Saturday.