Zanib Ahmad (Credit: Instagram/Zanib Ahmad)

UPDATED: 1/28/2013 – 4:01 p.m.

CHICAGO (CBS) — A North Side family is grieving after a woman and her granddaughter died, apparently of carbon monoxide poisoning.

Relatives told CBS 2’s Derrick Blakley the two women woke up feeling ill.

The only working carbon monoxide detector in the apartment building — a two-story building the family owns in West Rogers Park — was in the wrong place. It was in the basement, not far from the furnace, near an open window.

Assistant Deputy Fire Cmsr. Mark Nielsen said, “That could have created a draft that’s significant to keep C.O. away from that detector.”

When paramedics arrived Sunday morning, 18-year-old Zanib Ahmed was suffering seizures, and her 77-year-old grandmother Rasheeda Akhter was in cardiac arrest. Both died at Swedish Covenant Hospital.

Authorities are investigating whether a carbon monoxide leak caused the deaths. Autopsies were planned for Monday afternoon.

A stream of grieving friends and relatives visiting the building on Monday.

Akhter’s son, Shabbir Ahmed, said his mother was getting on in years, but still was in generally good shape.

“She was a beautiful person,” Shabbir Ahmed told CBS 2′s Susanna Song.

Family friend Fozia Anwar said it’s hard to believe the two women died from carbon monoxide poisoning.

“I can’t imagine how it happened. It’s very difficult,” she said.

When firefighters first arrived, they found very low levels of carbon monoxide near the furnace. However, over time, levels don’t have to be high to be deadly.

“CO poisoning is cumulative. It adds up. So, you get a little here, a little there, it keeps adding,” Nielsen said. “We believe it to be low-dose, long-term exposure.”

State law requires CO detectors within 14 feet of each bedroom. Fire officials said, if such detectors had been present near the bedroom, they would have saved two lives.

Fire Department District Chief Joe Roccasalva said, “If they were in proper places – as CO rises to the second floor, it may have been more abundant on the 2nd floor – that detector would have went off.”

Two other family members and five children ranging in age from 5 to 12 were taken to the hospital to be checked out but, Shabbir Ahmed said they’re all OK. He said it’s a tough time for the family of Pakistani immigrants, but their Muslim faith is seeing them through.

“We have to come in on this planet and one day we have to go back,” he said.

Zanib Ahmed was an honor student at St. Scholastica Academy, her family said.