Speaking alongside the mayor at a news conference, Police Colonel Oscar Perez said the person was detained “earlier this morning” and law enforcement officers are “not at this point” looking for anyone else in relation to the attack.

A gunman opened fire at the Ivy League school just after 4 pm (2100 GMT) on Saturday, with the school sending out an alert of “an active shooter near Barus and Holley Engineering.”

Two exams had been scheduled at the time.

Of the nine wounded one is in critical condition, seven are in stable condition and one has been discharged, Smiley said.

Police released 10-second footage of the suspect, seen from behind, walking briskly down a deserted street after opening fire inside a first-floor classroom.

The violence is the latest in a long line of school attacks in the United States, where attempts to restrict access to firearms face political deadlock.

The deadliest school shooting in US history took place at Virginia Tech on April 16, 2007, when South Korean student Seung-Hui Cho killed 32 people and wounded 17 others before taking his own life.