CHICAGO — Thirteen people were shot to death over the Labor Day weekend in Chicago, making it the deadliest holiday weekend of one of the deadliest summers the city has experienced in decades.

The police department also said the 13 were among 43 people who were shot over the weekend. Among those was a pregnant woman who delivered a nearly full-term baby after she was shot in the abdomen. The woman, whom police say was not the intended target, was listed in critical condition Tuesday. The infant’s condition has not been released.

The holiday weekend slayings come amid a dramatic spike in homicides. The department also said the weekend slayings pushed to 488 the total of slayings for the year — surpassing the 481 homicides that the police department said were recorded for all of last year. Nearly 230 homicides occurred in June, July and August — during which the city had 90 homicides, making it the deadliest single month in Chicago since June 1996.

Last week, the police department said it had officially increased the total of 2015 homicides to 481 from 473. Spokesman Anthony Guglielmi said the number increased to include victims who were shot in 2015 but who died this year, and because in some cases, death investigations had been upgraded to homicide investigations.

The number of shootings and homicides over the Labor Day weekend were both higher than the Memorial Day and July 4 weekends and it was the last of the summer holidays before the school year started Tuesday. The nine homicides that happened on Monday alone was one more for the entire Labor Day weekend last year.

Police have said the reasons for the uptick in homicides are tied to the easy availability of guns and gang violence. And Superintendent Eddie Johnson, as he done in the past, said that much of the gun violence is the result of lax gun laws in Illinois that allow those arrested on gun charges to be released from jail far sooner than in other states.

“I’m frustrated… that despite these weekends we still see repeat offenders get back out on the street far too soon,” Johnson told reporters on Tuesday.

Johnson, referring to reports that Chicago had more homicides than the combined total of the larger cities of New York and Los Angeles in August, said that those cities benefit from tougher gun laws. “If we had the gun laws they have we’d see violent crime cut in half,” he said.

Johnson also reiterated that most of the killings have been concentrated in neighborhoods on the city’s South and West sides that are plagued with high unemployment and poverty and where gang membership is particularly high.

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This story has been corrected to report that 481 homicides were committed in Chicago in 2015, instead of 473, due to a Chicago Police Department recalculation announced last week.