Mars in Libra and Denver, CO.

"At least 12 people have been killed and 38 injured in a shooting at a midnight cinema showing of the new Batman film near the US city of Denver, Colorado." Read
More-> BBC News

The planet Mars is considered to be in it's detriment in Libra, as Mars rules Aries (& Scorpio), the sign opposite Libra. So Mars is out of it's element, so to speak, and somewhat predisposed to cause problems when placed in Libra.

The actual chart for the ingress of Mars into Libra cast for Denver, CO. reflects the potential for explosive violence and hardship. Uranus (in Aries) was conjunct the Midheaven (most public point) and square a partile (in same degree) Moon-Pluto conjunction (8 Capricorn). As the Moon rules the Ascendant (Cancer), it's contact with Pluto and Uranus is particularly relevant to this location (Denver).

From United States

The Ascendant = 22.16 Cancer = Saturn 22.49 Libra. In other words, Saturn was in a partile square to the local Ascendant as Mars entered Libra. This tight and heavy aspect suggests possible hardship and separation, sometime during the transit of Mars through the sign of Libra.