Two brothers waiting at a train station on the Northwest Side of Chicago were violently attacked by a man wielding a machete on Monday after they refused to throw up gang signs.

In surveillance footage captured by the Chicago Transit Authority, seven people, including three teenagers, can be seen approaching the two victims, ages 26 and 17, as they were sitting on a bench at the Kedzie Brown Line station around 12:30 a.m., reports Fox News.

Police say that three 15-year-olds--two boys and one girl--along with four adults surrounded the brothers and demanded that they display gang signs. However, when they refused one of the suspects pulled out a machete and "held it to the throat of one of the victims," according to Cook County State's Attorney's office spokeswoman Tandra Simonton, reports the Associated Press.

Another juvenile pulled out a box cutter and pointed it at one brother, said Simonton.

Although the older brother tried to keep the situation from escalating further, "one of the defendants swung the machete at him ... causing a massive head wound," she said.

Eventually the group ran away when a train approached the station, but one of the victims was left with a huge gash on his head that required 30 stitches. He was treated at a local hospital and is said to be in stable condition.

Authorities with the Chicago Transit Authority say that the four adults were charged with one count each of robbery, aggravated battery and trespassing on CTA property on Tuesday. Meanwhile, the three young teens were charged with trespassing, and one of was charged with aggravated assault.

Police identified the man who swung the machete as 18-year-old Mario Elvira. He is being held in lieu on a $450,000 bond, while a second suspect, 20-year-old Kevar Preston, was also held in lieu of $450,000 bond. The other suspects, 23-year-old Jean Salvatierra and 19-year-old Kevin Ramirez are being held on a $250,000 bond each.

The adults were ordered to return to court on Sept. 15.