Chicago ? Melissa Cook’s family had traveled to Chicago from as far away as Florida to help her celebrate her 30th birthday on Saturday. But first, she and her cousin decided to go shopping downtown with their mothers.

The four women were in Cook’s car at a stoplight next to the John Hancock Center when a heavy piece of scaffolding broke away from the building and crashed to the pavement below, crushing the car and instantly killing Cook, 29, of Chicago, and her cousin, Jill Semplinski-Nelson, 27, of Olathe, Kan.

A third woman, Nanatta Cameron, 39, of Chicago, also was killed when a car she was in was crushed by the falling debris.

“It’s such a horrible, fluke thing  a freaky thing,” said Semplinski-Nelson’s cousin, Melissa Rohrer, of Merrillville, Ind., where Jill Semplinski-Nelson lived until about two years ago. She said her cousin had waved a pedestrian through the crosswalk just before the scaffolding fell. “It’s so terrible. The chances of this happening are a kazillion to one,” Rohrer said. “If they had just gone through (the light) themselves, they would have been alive.”

Investigators were working Sunday to determine what caused scaffolding to plunge from the 43rd Floor of the Hancock building at midafternoon Saturday as winds whipped through the area with gusts the National Weather Service said reached 58 mph. They pulled the scaffolding inside the Hancock at about 2 p.m., and began taking its lower panels apart, according to Cmdr. Tim Stokes of the Chicago Fire Department. Sunday night, authorities reopened Michigan Avenue and another street near the building. A third street near the building remained closed Sunday night.

City building officials also said they wanted to know whether the two scaffolding systems, which had been left in place almost halfway up the building, were properly secured at the time of the accident.

A city ordinance requires scaffolding systems to be locked in position when not in use. City Building Commissioner Mary Richardson-Lowry said Sunday she was not sure if or how the scaffolding was secured.

“It is doubtful the swing stage should have been up that high with the winds,” Richardson-Lowry said, adding it was too early to determine the main cause of the collapse.

“All we know right now is where it ended up,” she said.

The owner of the building, the Shorenstein Co., issued a statement saying it was “shocked and saddened by the tragic accident.”

The scaffolding had been erected so crews could complete building maintenance and cleaning, Fire Commissioner James Joyce said. A Shorenstein Co. spokeswoman said the cleaning was to take 1 1/2 years and that the work was about 70 percent completed.

Richardson-Lowry said the scaffold was owned by Beeche Systems Inc. of Skotia, N.Y., and was being leased by AMS Architectural Services, which was completing the maintenance work. AMS, in turn, contracted Prime Scaffolding of Bensenville to handle the rigging and move the company, according to Shorenstein Co.

Richardson-Lowry said AMS officials had not said whether they were operating the system Saturday. The city prohibits people from working on a scaffold in winds over 35 mph, she said.

The National Weather Service issued wind advisories Friday and Saturday, warning of high winds with possible gusts over 50 mph, meteorologist Allan Fisher said Sunday. He said some gusts reached 60 mph late Saturday.

Broken glass still littered the streets around the 100-story Hancock building, the city’s third-tallest building, which anchors the north end of the city’s busy Michigan Avenue shopping district. Many stores that usually are bustling on Sunday were closed.

Still-dangling sections of broken scaffolding suspended by cables from the top of the building had been secured on Sunday as crews worked to board up windows shattered when the platforms blew against the glass. In all, 66 windows were damaged, according to the Shorenstein Co.’s statement.

The scaffolding was “swaying around like a cat toy,” said Jerome Manansala, who works at a deli inside the Hancock building.

He said he and dozens of other bystanders had tried unsuccessfully to push the crushed cars out of the way as pieces of scaffolding dangled above and debris continued to rain down onto the street.

“In recent months that’s just been the spirit,” he said. “There’s a paranoia around, but along with the paranoia, there’s a sense of responsibility.”

Those injured included Semplinski-Nelson’s mother, Betty Lou Semplinski, 56, who was in fair condition at Cook County Hospital Sunday with a broken leg and hip, and Cook’s mother, Linda Demo, who was treated and released, Rohrer said.

A woman in her 50s was in critical condition Sunday at Northwestern Memorial Hospital, where four others had been treated and released, hospital administrator Shelley Williams said. She would not describe their injuries.

At Cook County Hospital, Michelle Whitaker, 24, was released, a hospital administrator said Sunday. A 53-year-old man was treated for minor injuries and released from Grant Hospital on Saturday, officials said.