"We face one of the most serious terror threat environments since the 9/11 attacks as foreign terrorist organizations continue to exploit the Internet to inspire, enable, or direct individuals already here in the homeland to commit terrorist acts," the bulletin, issued through the National Terrorism Advisory System, said.
An NTAS bulletin was first issued in December 2015 to "highlight the continuing threat from homegrown terrorists" and has been renewed three times previously with updated language.
Monday's bulletin, which is set to expire in November, includes new warnings on the techniques used by terrorists, like car attacks, that did not appear in the previous iteration.
"Terrorist groups are urging recruits to adopt easy-to-use tools to target public places and events," the bulletin reads. "Specific attack tactics have included the use of vehicle ramming, small arms, straight-edged blades or knives, and homemade explosives, as well as other acts such as taking hostages."
The reference to vehicle ramming comes after a series of attacks around the world in recent months where alleged terrorists used cars and trucks as weapons. In April, a suspected terrorist killed four pedestrians on a busy Stockholm street with a hijacked beer truck.
The bulletin also includes new language describing foreign terrorist fighters who may be "attempting to travel to the United States on visas, from visa-waiver countries, with the aim of attacking the homeland or inciting others within our borders to conduct attack."
To respond, DHS is "putting in place enhanced screening and vetting measures to detect travelers with potential terrorist connections," the bulletin says.
A top DHS spokesman told reporters last week that in recent months the department has made changes to vetting procedures that could impact foreign nationals traveling to the United States, though he declined to provide any details.
In a news release announcing the bulletin, Homeland Security Secretary John Kelly said his decision to renew it came "after careful consideration of the current threat environment and input from intelligence and law enforcement partners."
"We are in a generational fight against terrorist groups and those they inspire, and for us to protect our homeland we will need constant vigilance and clear focus on staying a step ahead of the enemy," he said.
Speaking at an event at the University of Chicago on Tuesday, Kelly pointed to strong community networks as another way to prevent radicalization.
"There's very few people that actually get radicalized," Kelly said. "As an American society, we have families, we have friends, we have churches, we have mosques and synagogues, we have community this and that, we have police and all, we have a great country -- problems for sure, but we have a great country. And all of that works to keep people from becoming a white supremacist or radicalized or something like that," he said.