From CNN:
International terrorism is a serious global issue, one that most leaders and citizens agree needs to be taken seriously. However, when the risk is boiled down to a matter of immigrants, refugees, and American citizens, the threat becomes infinitesimally small. According to a review by the CATO Institute, the chancesof an American dying in a terrorist attack committed by a foreigner in the US stands at about one in 3.6 million. The breakdown includes attacks over a 41-year period and includes the 9/11 attack, in which 3,000 people died. Once you narrow it to refugees and and illegal immigrants, the threat is even smaller
Even so, the statistics above don't tell the whole story. For instance, in 2016, the number of Americans killed in terrorists attacks in the US was unusually high because of the Orlando nightclub shooting, which claimed 49 lives and is considered an act of terrorism carried out by a Muslim extremist. However, it's important to remember that attack was also the deadliest terrorist attack -- and the deadliest mass shooting -- in the US since 9/11. On the other hand, the gun violence statistics do not include suicides by firearm, which usually hover around or above 20,000 per year.