Racially motivated violent extremists, such as white supremacists, and militia groups are two of the major security threats facing the US.
Agencies contributing to the assessment by the Office of the Director of National Intelligence included the FBI, the Department of Homeland Security and the National Counterterrorism Center.
The assessment said extremists who promote white racial superiority are most likely to carry out mass-casualty attacks on civilians.
Investigators say they also have potentially frequent communications with extremists abroad who hold similar ideological beliefs and each seeks to influence the other.
The agencies said that recent political and social developments “will almost certainly spur” some domestic extremists “to try to engage in violence this year”.
These include claims by Republican former president Donald Trump and his supporters about fraud in November’s US presidential election, restrictions related to COVID-19, fallout from the 6 January Capitol riot, and conspiracy theories.
Other domestic extremist categories which concern government investigators include animal rights and environmental activists, anti-abortion protesters, anarchists and people who call themselves sovereign citizens who “believe they are immune from government authority and laws”.
Citing the report, House Intelligence chairman Adam Schiff, a Democrat, said in a statement that while lone actors posed some of the most difficult challenges to detect, the violence also involved sophisticated cells and plots.
Senate Intelligence Committee chair Mark Warner, also a Democrat, said in a statement that social media platforms had facilitated online radicalisation, helping white supremacists, violent extremist groups and militia movements to recruit, organise and in some cases, coordinate across continents.
US far-right and white supremacist groups sharply stepped up their distribution of racist or anti-Semitic fliers, posters, banners and other forms of physical propaganda last year, according to a study released on Wednesday by the Anti-Defamation League.