Exposure to media coverage of mass violence has been shown to predict poorer mental health symptomology. However, it is unknown whether such media coverage can have ubiquitous effects on average community members, extending to biological and perceptual processes that underlie everyday decision making and behavior. Here, we used a repeated-measures design over the first anniversary of the Boston Marathon bombings to track participants' self-reported distress, their eye blink startle reactivity while viewing images of the bombings, and their ability to perceptually distinguish armed from unarmed individuals in a behavioral shooting task. We leveraged a computational linguistics method in which we sampled news content from the sources our participants most commonly self-reported reading, and then quantified both the extent of news coverage about the marathon and the affective tone of that news coverage. Results revealed that participants experienced greater current distress, greater physiological reactivity to threats, and poorer perceptual sensitivity when recent news coverage of the marathon contained more affectively negative words. This is the first empirical work to examine relationships between the media's affective tone in its coverage of mass violence and individuals' threat perception and physiological threat reactivity.