A man has been held without bail in connection with the assault of a boy in Newton on Monday. 

The suspect, Carlos Cruz-Aguilar, was arraigned on Friday. His lawyer requested a competency hearing and says his client has mental health issues. 

The suspect will return to court on March 28 for a dangerousness hearing. 

Cruz Aguilar is accused of assaulting a 12-year-old boy on March 21 at Lexington and River streets. 

Before the assault, the suspect was seen getting off a northbound MBTA bus on Lexington Street and Roberta Road, police said. 

The suspect allegedly grabbed the boy while holding a large kitchen knife in his hand. 

Police said the boy was thrown into the street and his cell phone was taken from him. 

A prosecutor on Friday said the suspect told the boy he knew where the boy lived and that he would kill him. 

Nick Rossi, who witnessed the incident, thought the struggle was friendly play.

"Obviously if I knew the guy had a kitchen knife, I probably would have stopped and tried to help him," Rossi said. 

Police say the driver who spotted the suspect recognized the reflective strips on the man’s pants and called police. 

At 9:23 a.m., the suspect was seen at a restaurant on Moody Street in Waltham near the Newton line, where he left the victim’s cell phone. 

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