Revenge of the Ninja

In Japan, the home of Cho Osaki (Sho Kosugi) is attacked by an army of ninjas, resulting in the slaughter of his entire family except for his mother (Grace Oshita) and his younger son, Kane (Kane Kosugi). When Cho arrives at his estate and discovers the carnage, the ninjas attempt to kill him as well, but Cho, a highly skilled ninja himself, avenges his family and kills the attackers. Afterwards, however, he swears off being a ninja forever and moves with his son and mother to Salt Lake City, where he opens an Oriental art gallery with the help of his American business partner and friend, Braden (Arthur Roberts), and his assistant Cathy (Ashley Ferrare).
One night, Kane accidentally drops and breaks open one of the dolls, exposing a white dust (heroin) contained therein. As it turns out, Braden uses the doll gallery as a front for his drug-smuggling business. He tries to strike a deal with Caifano (Mario Gallo), a mob boss, but Caifano and Braden cannot find common ground and eventually engage in a turf war. Braden, as a silver "demon"-masked ninja, assassinates Caifano's informers and relatives to make him cower down. The police are confused about the killings, and local police martial arts trainer and expert, Dave Hatcher (Keith Vitali), is assigned to find a consultant. Dave persuades Cho to see his boss and Cho attests that only a ninja could commit these crimes, but refuses to aid the police any further.[1]
In order to avoid payment for his 'merchandise', Caifano sends three men to rob the gallery. Cho happens to walk into the gallery while the thugs are loading the goods in a van, is attacked and responds with hand-to-hand combat. The henchmen escape in the van with Cho in pursuit, but he fails to stop the thieves from getting away. Meanwhile, Braden stealthily arrives to Cho's art gallery to find that it was just looted. Cho's mother and Kane both encounter him; Braden kills Cho's mother, but Kane manages to elude him. Cho, badly mangled, returns to find his mother murdered and his son missing.
In order to finish the last witness, Braden hypnotizes Cathy, who is in love with Cho, to find and bring in Kane. Cathy has some training in martial arts but little Kane had learned ninjitsu from his father, and they have a long fight until Cathy is defeated. When she recovers her senses, she contacts Cho and informs him both of Braden's treachery and that he is a ninja. Seeing his only remaining son in mortal danger, Cho breaks his devotion to non-violence and makes his way to Caifano's headquarters to stop Braden. In the meantime, Braden finds out about Cathy's betrayal and prepares to have her executed. Kane manages to free himself and Cathy, and the two inform the police.
Braden makes his final assault on Caifano and his organization, killing everyone he encounters. Eager to help his friend Cho, Dave also rushes to Caifano's headquarters but is ambushed by Braden, who mortally wounds him. Cho rushes to help his faithful friend, but the latter dies in his arms. Braden and Cho duel to the death on top of Caifano's skyscraper. After a long struggle, Cho manages to kill Braden and is reunited with his son and Cathy.

Question: who attacked the home of Cho Osaki?
Answer: army of ninjas