Two violent ambush robberies, one outside a University of Minnesota administration building and the other just off campus, occurred late at night over the weekend and prompted police to issue crime alerts.

A student suffered a chipped tooth in the Friday night attack while having various items stolen by four suspects, ­campus police said. The other crime, less than 27 hours later and about two-thirds of a mile north, involved one of four perpetrators threatening the victim with a gun, police said.

Three teens under 18 were quickly arrested in the Friday night attack, according to police. Another fled. No arrests have been announced in the later robbery.

Despite the similarities in the two crimes, "preliminary indications make it appear that they were carried out by different suspects," University Deputy Police Chief Chuck Miner said. "The descriptions of the suspects [in the two cases] are a little bit different." Minneapolis police spokesman John Elder also said that the two incidents appear to have been carried out by different people.

According to police:

About 11 p.m. Friday, a student was walking on the Williamson Hall Plaza when four males approached from behind. One struck the student in the face while they demanded his valuables.

The victim gave up his wallet, phone and headphones, and the four suspects fled. The ­student suffered a chipped tooth in the assault.

Roughly two hours later, university police stopped four juveniles who matched the suspects' descriptions provided by the victim. Three arrests were made, and one suspect fled and remains at large.

The next robbery occurred about 2:30 a.m. Sunday off campus on the footbridge that connects Van Cleve Park and SE. 8th Street, just north of several university athletic facilities.

A 23-year-old man from Sartell, Minn., unaffiliated with the school was walking on the footbridge when four males wearing ski masks came up from behind, threatened him with a gun and demanded his valuables. They took his wallet, cellphone and charger and fled.

Passersby recovered the man's possessions nearby, along with a BB gun.

Anyone with information about the off-campus crime can call Minneapolis police at 612-692-8477. For the Sunday robbery, calls can go to university police at 612-624-2677.

In late November, Minneapolis police warned U students to be on the alert after a string of violent robberies near the campus over a single weekend. In each crime, the victim was assaulted.

A rash of sexual assaults and sometimes violent robberies starting in the fall of 2013 and continuing well into the following February set the campus on edge and prompted university officials to heighten security, adding brighter lights, more police and new security cameras. Most of the crime, however, targeted students walking off campus at night.

Paul Walsh • 612-673-4482