A group of five suspects has been terrorizing riders on the J train, robbing them at knife-point, police said on Thursday.

In the first attack, a man approached a 25-year-old woman on the northbound J train, just before the 121st Street and Jamaica Avenue stop on May 8. He pulled out a knife and then grabbed her cash and MetroCards from her hands at about 2:40 p.m.

A group of four, comprised of three men and one woman, then attacked an 18-year-old boy just two days later, police said. They held a knife and punched the boy as the train was approaching 111th Street and Jamaica Avenue at about 1:50 p.m. They took his iPhone 6, and a shopping bag with a jacket, a Gucci belt and Jordan sneakers, police said.

Just minutes later, another suspect -- described as a man in his 20s with dreadlocks -- attacked a 47-year-old man near the Cleveland Street and Fulton Street stop. The suspect demanded the man's cellphone, and punched him when he refused. He then pulled out a knife and took it, police said.

Two of the other male suspects are described as being in their 20s, and one is about 6-feet-2-inches. The third male suspect is in his 30s and about 5-feet-4-inches, police said. The female suspect was carrying a black jacket, concealing the knife under it.