CHICAGO (CBS) — Police are warning businesses about a string of recent robberies at cellphone stores in and around the downtown area.

In each incident, the robbers walked into a cellphone store, grabbed display phones and threatened to stab employees if they tried to intervene, according to Chicago Police. The suspects then ran off.

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In one incident, one of the robbers displayed a knife, police said.

The robberies happened:

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• about 10:05 a.m. Dec.. 23 in the 100 block of West Randolph;
• at 5:53 p.m. Wednesday in the 200 block of South La Salle;
• about 3:20 p.m. Thursday in the 1300 block of North Sedgewick; and
• about 11:55 a.m. Friday in the first block of South Clinton.

Anyone with information about the robberies should call Area Central detectives at (312) 747-8384.

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(Source: Sun-Times Media Wire © Chicago Sun-Times 2017. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)