The shriek is instantly recognizable.

"We'll hear someone scream, and we know there is one out," said Alba Collazo, Philadelphia's chief deputy register of wills.

A mouse, that is. Or, worse, plural.

Throughout its 116-year history, City Hall has had its fair share of vermin, but employees who have been there for decades say the current mice population is exploding.

"It's infested. I've never seen them this bad," said Matt Myers, inheritance tax chief, who has worked in the Register of Wills office for 30 years.  Myers said that just last week, six to eight mice were caught in Room 180, the main reception area for the Register of Wills. Myers' office is across the hall.

The Department of Public Property, however, says there is no problem. Through a city spokesman, department officials declined to be interviewed for this article. Instead, spokesman Mike Dunn offered four one-sentence bullet-point statements that said in part: "There have been no reports from the pest control professionals indicating an increase in mice activity." And: "There has been no emails or verbal conversations indicating an increase, from the pest folks."

Still, the city's pest-control costs have more than doubled in the last four years. In 2014, the city paid $55,777. This past fiscal year, which ended June 30, the city paid $133,573 for pest control. "Increase in costs is due to bedbug treatments," Dunn said.

Myles Guevremont, general manager for Tri-County Pest Control, which has the pest control contract for City Hall, said he could not say whether there has been an increase in mice because this is the first year his company was contracted for pest control.

"We don't really have anecdotal data to say, compared to last year it looks worse," Guevremont said.

But for those working on the first and second floors, this spring and early summer have been a contrast to prior years.

City Commissioner Al Schmidt, who's been in office since 2012, said that his first-floor office had not had "any issues for a long time," but that within the last month, there had been a constant flow of mice in his office, and they were not there to register to vote.

"We have 10 different types of traps set up," he said.

Recently, Schmidt caught the whiff of decomposition from an unseen corpse. The smell seemed to come from a back closet.

"We tore it down to find it," Schmidt said. "It was behind the water cooler."

Collazo, who has worked in the Register of Wills office for more than 30 years, said that every time there is construction nearby, the mice tend to be more active.

"This has been our worst year that we've ever seen," Collazo said.

Experts say the problem likely has been exacerbated by people leaving food behind and the building not being properly sealed.

Martin Overline, president of Home Paramount Pest Control, which had the city's pest-control contract for several years until 2016, said the Department of Public Property needs to be involved in making sure there are not openings in the building to allow rodents in.

"Mice usually follow utility lines. … If the building is properly sealed, they can't travel along the lines," he said.

Carlos Hernandez, owner of Pest-A-Side, which until May had the largest pest-control contract with the city, blames the city for not having a designated eating area for employees.

"I repeatedly said to them: put everything in a garbage disposal and get rid of it every day," Hernandez said. "Don't eat at your desk. If you eat at your desk, of course mice are going to be around."

Guevremont, who now oversees pest control in City Hall, said that mice issues tend to be isolated within specific offices and are not building-wide: "The range of a mouse is very small. … They are only traveling 15 to 30 feet from where there is a nest."

The Register of Wills office has sticky traps set up all along the perimeter of the floorboards.

Collazo said sometimes she can hear when the mice are caught on the sticky traps. Once they are dead, she can smell it: "I'll call maintenance."

The customers who come in for birth certificates, marriage licenses, and services at the Register of Wills tend to be less scared than the employees who say they see the creatures almost daily.

"A woman was sitting there, and she just goes, 'Oh, there goes the man with the brown shoes and fur coat,' " one of the Register of Wills receptionists said, reenacting her shock at the time.

The second-floor press office, across the hall from Mayor Kenney's suite, was also inundated with mice earlier this spring and into early summer. Sticky traps were set up to no avail, despite multiple sightings. A mice inspector set up a spring-loaded trap he called "the neck-breaker." Two mice were caught on it within a week.

On the other side of the second floor of City Hall, Room 296, which houses the Civil Filing Center, has been closed this summer due to bedbugs and renovations.

Hernandez, the city's former pest control manager, called the pest situation in city-run buildings "very bad."

"Bedbugs are sucking blood all over the place," he said. "There's mice running all over the place in police stations and fire stations."

As for the mice in City Hall, Collazo said, she is considering getting an office cat.

"We would have to have a big one," she said.