CHICAGO — The killing of Chicago Police Officer Ella French brought to light one of the city’s most perplexing problems — how to slow the flow of illegal guns that are fueling the surge in violence.

The handgun used in French’s killing was allegedly bought by a straw purchaser in Northwest Indiana, then transferred to Chicago where it was used in her shooting.

Experts say many weapons used here travel a similar path.

It is estimated that 60% of the guns recovered in Chicago were originally purchased out of state, with Indiana, Mississippi and Wisconsin among the top suppliers.

“The flow of illegal guns into Chicago is absolutely a problem,” University of Chicago’s Kimberley Smith said.

Smith is an expert on underground gun markets.

“The police department recovers 10,000 firearms every year in the city of Chicago, yet there are zero…gun stores within city limits,” Smith said. “So, none of those guns originate here.”

There is hope that a new federal initiative, unveiled last summer in Chicago by Attorney General Merrick Garland, will help law enforcement crack down on illegal gun traffickers.

But undoubtedly challenges remain.

“People talk about this flow of guns,” John Lausch, Chicago’s top federal prosecutor, said. “Often times, we do see larger amounts. But typically, it’s three or four at a time. Maybe five or ten at a time. It’s really like the marching of ants sometimes. But it is a constant march. And that’s why we need to be persistent in going after them.”

According to records obtained by WGN Investigates, Chicago police seized more than 36,000 guns during a recent five-year span — more than police in Los Angeles, New York, Houston and other major U.S. cities.

Yet, shootings remain stubbornly high – up 66% from two years ago, according to Chicago police data.