Dearborn police also urged the public to step forward with any tips about the suspect, a man in dark clothes and a hood over his head who waved as he entered a Family Dollar store Monday night.
The body of a new employee, Joseph Orlando, 20, was discovered shot to death Tuesday morning, while assistant manager Brenna Machus remains missing. The motive was unknown.
"We believe she was abducted, absolutely," Police Chief Ron Haddad told The Associated Press.
Earlier, at a news conference, he said: "We as a community will never tolerate such a violent crime."
The face of the suspect can't be seen from the store's surveillance video, but Haddad hopes someone recognizes his walk or his clothing.
The man is waving on the video, although Haddad said he doesn't know if the victims knew him.
"You typically say hello to somebody if they're in eyesight. I can't make much more of it than that," the chief said of the wave.
Machus' mother, Lisa Machus, said she just wants her daughter back — "no questions asked."
"Brenna is a shining star. She's loved by many," Lisa Machus said. "If there's someone out there that has her, drop her off at a hospital. Let us know. We will come get her."
Jennifer Forte urged the public to call Crime Stoppers of Michigan, which is offering a reward. "Please find my best friend," she said.
Outside the store, Janet Post, 55, placed flowers and lit a candle to honor Machus and Orlando. She said she made a purchase from the pair on Monday.
"I was goofing around with him at the register. He waited on me, and she was sitting behind the counter," Post recalled, wiping away tears. "I walked out and had a good feeling about him. Both of them — they were so young. It's just terrible, terrible."
Machus has worked for Family Dollar for more than a year at different locations in the Detroit area. Orlando was hired just last week.
"He was ecstatic. ... He was smiling all the time. It's just senseless," Orlando's grandfather, Alan Konkel, said before stepping away at a news conference, overcome with grief.
Family Dollar regional vice president Rick Smith attended but declined to comment.
Haddad said detectives also want to talk to two women who were in the store between 7 p.m. and 7:30 p.m., before the suspect entered. He said they're not suspects but perhaps they could describe a car or what was going on in the parking lot Monday night.
Anyone with information can call police at 313-943-2241 or Crime Stoppers at 800-773-2587.