Detectives collecting evidence near Millbrae library after 13 yr old boy was stabbed today. Police say he's going to be ok. #ABC7News pic.twitter.com/5EAv2gGXQu — Lonni Rivera (@lonnirivera) June 10, 2017

Scattered belongings outside Millbrae library where 13 yr old boy was stabbed. Police arrested transient man and they say teen will be ok. pic.twitter.com/l1kAUrYHvN — Lonni Rivera (@lonnirivera) June 10, 2017

MILLBRAE, Calif. (KGO) -- A Millbrae teenager is recovering after police say he was stabbed outside the local library on Friday.San Mateo Sheriff's deputies say his alleged attacker is in custody and that detectives believe the stabbing was completely random.In Millbrae, schools ended Friday classes early. Then students went to the library. It was a normal routine that turned frightening after the 13-year-old became a victim of a stabbing attack."It's just appalling, it really is," said library visitor Ann Hornstein. "It makes you frightened for your children; it makes you frightened for yourself."Detectives say the boy was hanging outside of the library with his friends, when a 40-year-old transient man walked up to him."Without any sign or reason, he just began to stab him with what we believe to be a pocket knife," said San Mateo County Detective Salvador Zuno.They say the man stabbed the boy multiple times in both arms."The teenager was shocked, you know, he was panicking, he didn't know what was going on, he didn't know why he had been attacked," Zuno said.Police officers at a substation next door rushed to the scene. Officers administered first aid on the young victim before he was taken to the hospital. In the meantime, police took the man into custody. And detectives say that, strangely, the suspect hung around."After the attack, he didn't flee," said Zuno. "He stayed in the area. So on top of that, it's another unusual circumstance."The teenager is a Taylor Middle School student. He underwent surgery and is expected to recover.As for his alleged attacker, that man is in San Mateo County Jail. Detectives say they're trying to figure out who he is and why he randomly attacked a kid.