There are a lot of things to consider when characterizing a group as a terrorist organization. Naturally, just by the general definition of terrorism and the FBI's description of domestic terrorism, we can see a great deal of violence coming from Antifa groups against people who peacefully support the United States President, Donald Trump, and these attacks are inspired by political motives. These attacks include throwing "milkshakes" at people, throwing cement at commuters, and outright violent confrontations with other protesters where weapons such as hammers, crowbars and other blunt objects are used. For some Antifa members, the political motive is due to having a "white nationalist" president and opposing him, others are more directly opposed to our current economic system as they support blatant anti-capitalist values. Of course, these attacks on people are not justified but are they uniform or are they organized?
We can view some of their attacks, where they unjustly attacked people, as an act of domestic terrorism, not extreme violent terrorism that has led to mass murder but politically-inspired assaults. Generally speaking, most individuals who claim they are Antifa are just riding the bandwagon. This doesn't mean their concerns are not just but it does suggest that they are easily influenced by organized Antifa rhetoric. Not every single member of the Antifa participate in these attacks, but many within their ranks condone these attacks. The 'organized' Antifa members or those who allegedly may have gained their funds from rich donors are the primary suspects for the violence and this has naturally created a wedge between the moderate left and far-left, even within the Democratic Party. Most Antifa groups tend to not be organized; in fact, many are locally grown communities and they tend to be "leaderless" even though they do have public speakers who represent said local Antifa groups.
These leaderless groups are not classified as a registered organization, and if the US Federal authorities were to try and declare them as such, they may inadvertently place innocent Antifa protesters in the crosshairs of a nasty political war. It would be tough to classify them as a terrorist organization with no leader and no real chain of command. This doesn't suggest that they can't be arrested but it's a slippery slope to classify them all as a domestic terrorist threat, and if we do classify them as such their ranks will grow as they continue to spread misinformation through social media sites.
There is no question that Antifa engages in terrorist actions, and so far they have not directly resulted in mass murder as a collective but I think it's important to monitor the groups further as a possible Homegrown Violent Extremist (HVE) threat. I do consider them to be far-left wing terrorists, however, we must be certain to separate the militant Antifa group from the peaceful protesters who oppose Donald Trump. We need to be certain of whom we are classifying and if we target them all, this will lead to a far worse unintended consequence.