By Nancy Loo

Chicago, IL (WGN/CNN) - Employees at a Chicago hospital are on alert after two doctors were attacked walking home from work.

Northwestern Memorial Hospital sent a memo to staff with a warning.

By now, most hospital employees have seen the memo.

"I'm surprised. I'm shocked that it happened, but I don't think it would happen again, I guess," said hospital employee Bridgette Reiner.

[Reporter]: "Why not?"

"I've lived here a long time and it's just, the area is nice. It's usually safe," said Reiner.

While Streeterville is perhaps one of the safest neighborhoods by day, late night hours are of particular concern.

Northwestern security issued the memo to employees after two doctors were attacked while walking home.

On Sunday around 10 p.m., one doctor was surrounded by a mob of up to 20 people on Dewitt Street.

The memo stated that "As a group of young men and women passed, some circled back and punched the doctor in the back and used an object to hit him in the forehead."

Another doctor was attacked a week and a half ago on Chicago Avenue. He was punched in the face after also being accosted by a group of young people.

Security and police patrols have been beefed up and the hospital campus is surrounded by surveillance cameras, but all employees are now urged to be on alert and to walk in groups, especially after dark.

"We have breaks and stuff during the night and we're always walking back and forth to the buildings and we feel safe," said nurse Susie Rubmer.

"Late at night you want to be careful and we have escorts for that, but, I just feel that it's a safe place," said Reiner.

In both incidents, the doctors were not seriously hurt and nothing was taken.

WGN/CNN