Law-enforcement officials boosted their appeals to the public to be their eyes and ears as they heighten their focus on random assaults on less likely targets, prompted by the weekend attacks in New York City, New Jersey and Minnesota.

The San Francisco Bay Area’s main commuter rail line, Bay Area Rapid Transit, “increased the frequency of the ‘see something, say something’ message on platform signs and the PA system,” said Alicia Trost, a BART spokeswoman.

Police in Los Angeles urged the public to download a smartphone app that allows them to report suspicious activity. “We ask everyone to stay vigilant,” Police Chief Charlie Becksaid.

In Chicago, the Office of Emergency Management and Communications said people should dial 911 if they saw something unusual.

Police agencies also deployed more officers at transit stations for the Monday commute and at events expected to draw crowds, including a technology conference in San Francisco.

For years, law-enforcement officials worried about large-scale terrorism at marquee events like championship football games and high-traffic tourism sites. Unlike terrorist groups like al Qaeda, which targeted major landmarks, Islamic State tells its followers to travel to Syria or, if they can’t, to kill people wherever they are.

That has led to greater concerns about attacks in scattered locations across the U.S., such as the mass shooting at a county employee gathering in San Bernardino, Calif., last December that killed 14, and a gunman’s assault at an Orlando, Fla., nightclub in June that left 49 dead.

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