The Boston Police Department is taking heat from civil liberty groups for plans to spend up to $1.4 million on new software that scours social media and the internet for potential threats.

The attack Monday on the Ohio State University campus is just the latest illustration of why local law enforcement authorities need every tool they can muster to stop terrorism and other violence before it starts, according to Boston Police Commissioner William Evans.

Monitoring technology can quickly mine the internet, from chat rooms to social media to blog posts, for certain keywords and phrases. It can track postings in a certain geographic area, send alerts to police about potentially dangerous postings and more. Law-enforcement officials say the technology allows them to more quickly and efficiently spot possible red flags in near real-time.

Officials say the Ohio State suspect may have been inspired by the Islamic State terror group. A Facebook post by the suspect Abdul Razak Ali Artan before the attack suggested he was angry over what he perceived as mistreatment of Muslims, but didn’t express loyalty to a specific group or ideology, according to people familiar with the case.

Sharing Islamic State propaganda by itself isn’t a crime. But if someone is making threatening posts, police might then use informants or other means, including more surveillance or seeking court permission to monitor phones or computers, to gauge how serious the person is. “The more you know about someone, the more you can make informed decisions about how many resources to put into those people,” said Edward Davis, the Boston police commissioner during the 2013 Boston marathon bombings.

It is hard to say whether monitoring would have made a difference in thwarting the Boston bombers, who were allegedly motivated by online anti-U.S. jihadist teachings, because the bombers weren’t very active on social media, said Mr. Davis.

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