TAMPA — Amazon is racing to complete a massive warehouse in southeastern Hillsborough County in time for the holiday shopping rush and likely will end up hiring even more people than the 1,000 originally planned.

Jim Ford, interim head of the county's building and construction services, said Tuesday that crews have erected the 1.1 million-square-foot building and are working on the interior. He expects that the building will receive a partial certificate of occupancy in the late fall and could start processing orders by Christmas.

"They are on a very accelerated deadline,'' he said. "At times they are working 24 hours a day.''

Based on his discussions with project officials, Ford expects the online retail giant to hire about 1,100 workers initially and then expand to up to 2,500 people — permanent and seasonal — once the center is fully operational.

The Seattle-based retailer hasn't scheduled any job fairs yet but could begin hiring people in late summer, he said. Employees will be needed to stock merchandise, set up the computer network and train office personnel.

Atlanta-based contractor Conlan Co. started the foundation work in November, about a month after Amazon confirmed plans to open "fulfillment centers'' in Hillsborough County and Lakeland for processing, packaging and shipping orders. It laid the slab in January and put up the walls in March. Concrete walls were poured on site and lifted upright using cranes.

To streamline the process, the company hired a private contractor to do the inspections and submit them to the county for approval, a common practice for large, complicated projects. Even then, county building officials have been out to the site once or twice a week overseeing work, Ford said. Two county workers spend about half of their time keeping up with the paperwork.

Located near Interstate 75 off State Road 674 in Ruskin, the Hillsborough center will process small items like books and CDs. The Lakeland center, which is also under construction, will handle larger items, from kayaks to TVs.

The Ruskin center will eventually use robots to locate order items and take them to a warehouse worker for further processing. Amazon acquired Kiva Systems Inc. in 2012 for $775 million and has been using its robots at newer centers to replace miles of conveyor belts and hundreds of runners. The robots will be phased in over the first few years.

Amazon has taken a no-frills approach to the building, Ford said. There are no fancy corner offices, cafeterias or workout centers. It's a big box with steel beams, concrete floors and mezzanines for different processing areas.

"It's the size that's impressive,'' he said. "It's cavernous.''

At more than a million square feet, the warehouse is about 10 times the size of an average Home Depot. To ensure employees don't have to walk a long distance while on break, there are eight break rooms throughout the building. The goal is that when employees go on a 15-minute break, they won't have to walk more than two minutes to get to a break area.

Ford said Conlan and Amazon, while tight-lipped about their projects, have been easy to work with. Because so many distribution centers have been built in the past few years, project officials have been able to anticipate issues and questions that might arise.

"They have immediately responded to any concerns we have had,'' he said. "We've had no issues with their design team. They have been great.''

Amazon didn't respond to messages about the project.

Amazon has been aggressively opening distribution centers nationwide to speed up delivery times and had been looking quietly for sites in Florida for a few years. Both Hillsborough and Polk counties offered Amazon millions in tax incentives over the next several years if the company creates hundreds of higher-paying jobs.

Establishing a physical presence in Florida means the online retailer will have to start collecting sales tax on items ordered in the state. Amazon customers currently aren't charged the tax, frustrating small retailers who have long complained that Amazon has an unfair price advantage.

Susan Thurston can be reached at sthurston@tampabay.com or (813) 225-3110.