The US Department of Homeland Security (DHS) has warned of an increase in extremist content and threats against US lawmakers in the days leading up to the anniversary of the 6 January insurrection, according to a memo obtained by the Guardian.

The memo, sent on Thursday to state and local law enforcement, said that DHS had no indication of a specific and credible plot, but that the agency and the FBI had “identified new content online that could inspire violence, particularly by lone offenders, and could be directed against political and other government officials, including members of Congress, state and local officials, and high-profile members of political parties, including in locations outside of [Washington DC]”.

John Cohen, the head of DHS’s office of intelligence and analysis, outlined a range of content on “extremist related platforms” that was concerning.

In one instance, an “unknown individual” posted a video online listing 95 members of Congress who, the video claimed, were involved in voting to certify the “fraudulent” presidential election, echoing far-right misinformation that has spread since last year. The video called for the Congress members to be hanged in front of the White House and was posted on a forum that hosts QAnon conspiracy theories and was reposted by Telegram users and on blogs.

The video was viewed more than 60,000 times across platforms, the memo said. Cohen also warned of a separate posting that referenced 6 January “as an appropriate day to conduct assassinations against named Democratic political figures, including [the president], because of the perceived fraudulent election”.

The US Secret Service, the Capitol Police and the DC Metropolitan Police agencies were aware of the online activity and have initiated investigations “as appropriate”, Cohen wrote in the memo, adding that the Federal Protective Service had also expanded patrols in and around federal facilities across the US. The memo comes amid reports of a sharp rise in threats against lawmakers over the last year.

DHS had recently warned that the US was facing a heightened threat environment across the country, as threat actors “continue to exploit online forums to influence and spread violent extremist narratives and promote violent activity”. The agency said it would continue to monitor the uptick in threats in the coming days.

After federal authorities were criticized last year for failing to act on warning signs on social media before the 6 January attack, DHS established a new domestic terrorism branch in its intelligence office, which has been focused on expanded tracking of online threats.

Earlier on Thursday, Joe Biden denounced Donald Trump for spreading a “web of lies” about the 2020 election and accused Trump and his allies of holding a “dagger at the throat of American democracy”.