The massive hacking campaign disclosed by U.S. officials this week and tentatively attributed to the Russian government extended beyond users of pervasive network software that had been compromised.
Another major technology supplier was also compromised by the same attack team and used to get into high-value final targets, according to two people briefed on the matter.
The FBI and other agencies have scheduled a classified briefing for members of Congress Friday.
The Department of Homeland Security said in a bulletin on Thursday the spies had used other techniques besides corrupting updates of network management software by SolarWinds, which is used by hundreds of thousands of companies and government agencies.
"The SolarWinds Orion supply chain compromise is not the only initial infection vector this APT actor leveraged," said DHS's Cybersecurity and Infrastructure Security Agency, referring to "advanced persistent threat" adversaries.
CISA urged investigators not to assume their organizations were safe if they did not use recent versions of the SolarWinds software, while also pointing out that the hackers did not exploit every network they did gain access too.
CISA said it was continuing to analyze the other avenues used by the attackers. So far, the hackers are known to have at least monitored email or other data within the U.S. departments of Defense, State, Treasury, Homeland Security and Commerce.
As many as 18,000 Orion customers downloaded the updates that contained a back door. Since the campaign was discovered, software companies have cut off communication from those back doors to the computers maintained by the hackers.
But the attackers might have installed additional ways of maintaining access in what some have called the biggest hack in a decade.
For that reason, officials said that security teams should communicate through special channels to ensure that their own detection and remediation efforts are not being monitored.
The Department of Justice, FBI and Defense Department, among others, have moved routine communication onto classified networks that are believed not to have been breached, according to two people briefed on the measures. They are assuming that the nonclassified networks have been accessed.
CISA and private companies including FireEye, which was the first to discover and reveal it had been hacked, have released a series of clues for organizations to look for to see if they have been hit.
But the attackers are very careful and have deleted logs, or electronic footprints or which files they have accessed. That makes it hard to know what has been taken.
Some major companies have issued carefully worded statements saying that they have "no evidence" that they were penetrated, but in some cases that may only be because the evidence was removed.
In most networks, the attackers would also have been able to create false data, but so far it appears they were interested only in obtaining real data, people tracking the probes said.
Meanwhile, members of Congress are demanding more information about what may have been taken and how, along with who was behind it. The House Homeland Security Committee and Oversight Committee announced an investigation Thursday, while senators pressed to learn whether individual tax information was obtained.
In a statement, President-elect Joe Biden said he would "elevate cybersecurity as an imperative across the government" and "disrupt and deter our adversaries" from undertaking such major hacks.