PHILADELPHIA (CBS) — Philadelphia police are asking for the public’s help locating missing 20-year-old Jimmy Mao, who they believe may have been kidnapped. Police say Mao was reported missing on Jan. 2, but was last seen on Dec. 29, 2019 at his residence on the 5800 block of Angora Terrace, where his sister says he lived with foster parents.

Despite her grief, Niki Mao believes her brother is okay.

“I had a dream that he is still alive. Something is telling me that he is still alive,” she said.

Police say that on the morning of Jan. 7, a family member began receiving text messages from an unknown person, saying they know where Mao is and demanded a ransom.

“I think he got kidnapped,” Niki Mao said.

Police have not been able to verify the authenticity of the ransom demand.

“We’re not sure if a hoax is being perpetrated against the family,” Philadelphia Police Capt. Mark Burgmann said during a press conference Friday afternoon. “Also, the nature of the demands that were made kind of does not seem to be very serious. However, he has not been seen in two weeks and we take these cases very seriously, so we are erring on the side of caution.”

Police say the family member responded to those texts but they have not paid anything. Police say they have not heard from the person demanding the ransom since Thursday.

“And then the texts will stop. They’ll stop and start again, there doesn’t seem to be any urgency at all,” Burgmann said.

Niki shared several of the demands and messages with Eyewitness News.

“They want $15,000,” she said.

The person who allegedly has Mao says “he is a bit sick now,” in one message, adding he should go to the hospital for proper care after the money is paid.

“There are very little specifics. There is no deadlines, there are no specifics on where to deliver money or when or how to go about that. There is none at all,” Burgmann said.

So Burgmann is asking for the public’s help tracking down Mao and hopefully reuniting a heartbroken sister with her brother.

“I want to tell him I love him and I want him to come home. All the family misses you. It doesn’t feel the same at all,” Niki said.

Police say Mao is 4-foot-11 and weighs 100 pounds.

If you have any information on his whereabouts, contact police at 215-685-3264.

CBS3’s Greg Argos contributed to this report.