Prosecutors said 18-year-old Sincere Williams was wearing two sets of clothes when he walked into the Walgreens at 1372 N. Milwaukee Ave. just after 9:30 a.m. Sunday and stabbed employee Olga Calderon 10 times with a knife.
He allegedly took off the extra clothing after leaving the store. Calderon was pronounced dead inside the Walgreens.
"He approached the victim, Olga Calderon, who was just stocking shelves at Walgreens, grabbed her around the neck," Area 5 Commander Eric Winstrom said. "It appears that she tried to push away from Williams, and he began stabbing her multiple times, causing her death."
WATCH: Chicago police announce charges, new details on motive behind stabbing
Police said co-workers discovered the 32-year-old dead in a pool of blood just before 10 a.m. Sunday.
"I don't really know what to think," Walgreens employee Jennifer Montanez said. "Very numb. It's hard to process, even if it's not someone that you particularly know."
There were no eyewitnesses, according to detectives. But detectives went through 30 minutes of surveillance video, tracking William's steps before and after the stabbing, prosecutors said.
"Williams discarded a knife and clothing in a nearby yard, which detectives recovered," Winstrom said. "We were able to determine that Williams entered an apartment complex in the 1400-block of North Sedgwick in the 18th District. With the assistance of 18th District tactical personnel, we were able to arrest Sincere at his apartment without incident."
Prosecutors said his mother later drove him to the hospital to be treated for an injury to his left hand that happened during the stabbing. Prosecutors assume robbery was the motive, but can't be certain; nothing was taken from the store. They said "sometimes people just do evil things."
"One of my best friends in the world works at a Walgreens and I am always afraid for her, now even more so," said customer Leandra Czech. "There is just too many crazy people out there and it is just ridiculous and this should never had happened."
"I am scared to go anywhere. I don't take my son anywhere. You never know, anywhere you go anything can happen," said Stephanie Burke, Chicago resident.
Williams has been charged with one felony count of first-degree murder. His attorney said Williams suffers from schizophrenia and seizures.
Police believe Williams is also responsible for an earlier robbery at this same Walgreens and at another location, possibly with the same knife and mask he allegedly used Sunday.
Police don't believe Calderon was targeted for any reason. They said she was just a young working mother trying to make a living who never made it home to her family.
Crisis responder Andrew Holmes said Calderon is the mother of a 3-year-old boy and an 11-year-old girl. Her family is struggling to come to grips with the loss.
"Her daughter is taking it really hard," Holmes said. "The younger baby doesn't understand yet what's actually happened, but the daughter, she knows."
"It makes me really sad we are in a place in society where we are worried about going home, going to work and we don't know if we're going to come back, we don't know if we're going to upset someone. We just don't know," Montanez said.
Walgreens said extra security was in place as the store reopened Wednesday morning.
A makeshift memorial outside the store continues to grow for Calderon. People bringing flowers, candles and balloons. Customers describe Calderon as pleasant and courteous.
Walgreens spokesperson Phil Caruso said in a written statement: "We are grateful to Chicago police and law enforcement for their efforts and will continue to work with authorities. The safety of our customers and team members is our top priority. We are continually evaluating our security measures in all our stores. The store is expected to reopen tomorrow morning with additional security measures."