CHICAGO — The last straw that convinced Chicago native Marissa Marshall it was time to move away was hearing dangerously close gunfire in the rough neighborhood where she found affordable housing.

The 29-year-old, who's pregnant with her fifth child, relocated about three years ago to a St. Louis suburb where she more easily found jobs and a home where she felt it was safe to send the kids outside.

“I have boys, and I didn't want to raise them in that environment,” Marshall said. “It is easier to go outside of Chicago to get help than stay in Chicago.”

New census data released Thursday shows she's not alone, with the Chicago area losing more residents than any other U.S. metropolitan area.

The continuing decline coincided with other Midwestern areas, including St. Louis and Cleveland, as the South and Southwest regions of the country saw gains. Two Texas metropolitan areas — Dallas and Houston — reported the biggest numeric increases between July 2015 and July 2016, adding more than 100,000 residents each.

There are wide-ranging reasons for the shifts, from families' concerns about violence and schools to dwindling immigration and fertility rates. But demographers said Thursday's data also suggest the reanimation of a trend that paused during the recession — of Americans on the move from the Snow Belt to the suburbs of big cities, and to the Sun Belt.

The Chicago area, which includes surrounding communities in Illinois, Indiana and Wisconsin, lost more than 19,500 residents in a year's time. Cook County, which encompasses Chicago, also led all counties in population drops, with roughly 21,300 fewer residents. Trailing were Michigan's Wayne County, where Detroit is located, with roughly 7,700 fewer residents; and Baltimore County, which lost more than 6,700 residents.

Meanwhile, Arizona's Maricopa County had the highest annual population increase, gaining over 81,000 residents, followed by Harris County in Texas and Nevada's Clark County.

Families leaving Chicago cite the nearly broke city school system that has closed over 50 schools since 2013 and a soaring violent crime rate with more than 760 homicides last year, the most in two decades. City demographics experts add in economic trends such as fewer entry level jobs and a sagging industrial core, along with the dismantling of dense neighborhood-based public housing.

Lower immigration rates also have impacted the Chicago region's dwindling population. Immigration, particularly from Mexico, was the key factor behind most of Chicago's population growth in the 1990s.

Kellman reported from Washington.