(CNN) — The Department of Homeland Security recently warned of potential domestic violent extremist acts related to the anticipated lifting of a Trump-era border restriction, according to a memo obtained by CNN, just days before the termination of those limits was halted by the Supreme Court.

"So far, we have observed calls for attacks targeting primarily migrants and critical infrastructure, but our insight into DVE plotting is constrained by these individuals' use of online security measures to limit exposure to law enforcement," reads the memo, dated December 23.

Threats against the United States are increasingly unpredictable and complex amid a host of conspiracy theories, foreign interference, and calls for violence.

Grievances over immigration policy and animosity toward immigrants have previously fueled extremist acts, including the 2019 Walmart shooting in El Paso, Texas, that killed 23 people and left another 23 wounded. Authorities said at the time the accused shooter drove to the West Texas border city with the sole intent of killing immigrants and Mexicans.

The December memo obtained by CNN focuses on the lifting of a public health authority known as Title 42, which has been a source of tension between Democrats and Republicans and linked to a surge of migrants at the US-Mexico border.

Since the onset of the coronavirus pandemic, the public health order has allowed officials to turn away thousands of migrants encountered along the US southern border, marking an unprecedented break from previous protocols. The Biden administration was initially on track to end Title 42 on December 21 to comply with a lower court order.

In anticipation of the termination of Title 42, DHS observed chatter about potential violence directed toward migrants and other targets. The bulletin, from the department's Office of Intelligence and Analysis, is dated four days before the Supreme Court said Title 42 will remain in effect while legal challenges play out — a process that will take months.

The December memo cites violent tactics discussed among social media users against migrants, including "firearms attacks, the placement of land mines along migration routes, and luring migrants into trailers to poison them with gas." Other users discussed shooting electrical substations near the US-Mexico border, likely to disrupt immigration facilities.

"We have not previously observed calls for substation attacks in response to immigration-related concerns, and these recent discussions may stem from widespread media coverage of recent attacks against other substations across the United States, particularly in Moore County, North Carolina," the memo reads.

CNN has reached out to DHS for comment.

DHS also acknowledges in the memo that domestic violent extremists have historically cited "immigration-related grievances" to justify violence and notes that militia groups have previously interfered with border operations.

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