LAGUNA BEACH, Calif.— Amazon.com Inc. has more than 10,000 employees working on its Alexa virtual assistant and the Echo devices it powers, the executive in charge of the business said, double the staff in that division a little more than a year ago.

Speaking at The Wall Street Journal’s WSJ Tech D.Live conference on Tuesday, Dave Limp, senior vice president of Amazon Devices, outlined new areas where Alexa can expand in the coming years, including in offices, cars and hotel rooms.

Amazon is investing to rapidly expand its workforce as it seeks to dominate areas of cutting-edge technology such as artificial intelligence. The company says it plans to hire some 50,000 workers across two new hubs it plans to build in New York City and Northern Virginia.

Amazon announced its decision Tuesday on those two locations, after its yearlong review of possible cities to establish a second headquarters. Mr. Limp said Amazon picked them because of the availability of talent.

“The tie went to where we could recruit and where people would want to live,” Mr. Limp said.