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.

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

The robberies happened:

• 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.

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 This material may not be published, broadcast, rewritten, or redistributed.)