They were thinking that her dad died of a heart attack, but after more family members passed they knew it was something else.
“At first, doctors suspected he'd had a heart attack…They were so grief-stricken that they had a headache,’ Kasia recalls. ‘They needed something to calm themselves down.’ So both of them took Extra-Strength Tylenol from the bottle her dad had bought earlier that day. Her uncle collapsed on the kitchen floor of Kasia’s home. Her aunt collapsed in the living room soon afterward.” (Shoichet).
He took some Tylenol before going to bed and later he wouldn't wake up. When they got to the hospital he died and doctors thought it was a heart attack. Family members were asking Karia what happened but she didn't know. Her uncle and new bride were so shocked and grief-stricken they both had a headache and needed to calm down. So they took the Tylenol Karia’s dad bought earlier. They later collapsed in their house and were all rushed to the hospital. They were all probably really scared knowing the fact that 3 family members had died within 24 hours and still had no explanation why.
It took people a month to figure out the connection of the deaths to Tylenol, with the issue growing throughout the month.
“It was at this point, early October of 1982, that investigators made the connection between the poisoning deaths and Tylenol, the best-selling, non-prescription pain reliever sold in the United States at that time.”(Dr. Howard Markel)
This quote shows that it took a month for the connection of the deaths to Tylenol to happen. As earlier in the source it says that the deaths happened in September. And, because this issue kept snowballing throughout the month, the pressure to find the cause was increasing. One family had multiple members dying of similar causes around the same time, all of them taking the Tylenol.