City Building Commissioner Norma Reyes said the city was unable to find a construction permit for the porch, which was built in 1998.
Officials found permits for other repairs at the building that year, but Reyes said it was unclear whether the porch exceeded the scope of those documents.
"Thus far, there is no evidence of any criminal activity whatsoever," Police Superintendent Terry Hillard said.
For Simon Rasin and other partygoers crammed onto a third-floor porch on Chicago's North Side, there was no warning when the floor caved in, sending dozens of people hurtling down in an avalanche of wooden planks.
"I fell through both the second and the first floor decks into the basement area in just a pile of bodies," said Rasin, a University of Chicago law student whose friend, Henry Wischerath, was one of the 12 killed. At least 57 others were injured, some critically.
As authorities searched for clues into the early Sunday tragedy in an affluent Chicago neighborhood, survivors described a harrowing scene: bodies stacked upon bodies, trapped under rubble, some victims moaning for help.
"There were people covering me. It was pitch black and people were yelling, 'I'm dying.' I was assuming I was going to die," said Natalie Brougham, 22. "I guess I got lucky and only had two or three people on top of me."
By Sunday evening, workers had torn the porch down. Lumber and red plastic cups littered the alley behind the building.
As many as 50 people, most of them in their early 20s, had squeezed onto the apartment porch for a party in the Lincoln Park neighborhood when the floor fell at about 12:30 a.m. Sunday.
Seven men and five women, most of them apparently on the porches directly below, were sandwiched between the falling floors and killed, said Larry Langford, spokesman for the city's Office of Emergency Management.
"There was chaos," Chicago Fire Commissioner James Joyce said. "There were people screaming and crying in the alley."
Partygoers who had been safe inside the apartment said they tried to rescue their friends from the pile of lumber and bodies, while people poured out of a nearby tavern to help.
"They were bloodied and covered in rubble, their clothes were ripped. Women were looking for husbands, men were looking for wives. It was horrible," said Geraldine Schapira, 33, who lives nearby.
Eleven people were pronounced dead at the scene, and the Cook County Medical Examiner's office confirmed that a 12th person was dead on arrival at hospital.
Most of the people at the party were friends in their early 20s, many of them graduates of New Trier High School in Chicago's northern suburbs, said Fina Cannon. She had been in the apartment's kitchen, looking out at the porch when it gave way.
"All of a sudden I saw all these heads going down," Cannon said. "The floor just dropped out from underneath them. They all went down in unison."
"It was simply a case of too many people in a small space," Joyce said. He urged people to be careful about safety, particularly with the upcoming July 4 holiday.
Authorities released the names of all the victims: Wischerath, 24, of Buffalo, N.Y.; John Jackson, 22, of Kansas City, Mo.; Katherine Sheriff, 23, of Chicago; Eileen Lupton, 22, of Lake Forest; Shea Fitzgerald, 19, of Winnetka; Muhammed Hameeduddin, 25, of Chicago; Margaret Haynie, 25, of Evansville, Ind.; Sam Farmer, 21, of Winnetka; Eric Kumpf, 30, of Hoboken, N.J.; Robert Koranda, 23, of Naperville; and Kelly McKinnell, 26, of Barrington; and Julie Sorkin, 25, of Glenview.