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WASHINGTON (Reuters) - Alphabet Inc's GOOGL.O Google dominates the growing U.S. search ad market but Amazon.com Inc AMZN.O is growing fast, according to data from the market research company eMarketer released on Tuesday.

EMarketer data showed that the U.S. search ad market will grow by nearly 18% in 2019, and Google will have 73.1% of that market and revenues of $40.33 billion. EMarketer said that Google’s share would decline to 70.5% by 2021.

Google faces investigations by the Justice Department, House of Representatives Judiciary Committee based on allegations that it abuses this dominance in ways that violate antitrust law.

The data also showed that Amazon, which overtook Microsoft Corp MSFT.O for the No. 2 spot in 2018, and has 12.9% of the market, boosting search revenue to $7.09 billion.

“Polling suggests that most product searches now begin on Amazon, causing the No. 2 search player to grow rapidly and steal share from its larger rival,” said eMarketer.

Rounding out the top five is Microsoft at 6.5% of the market, Verizon Media, which includes AOL, at 2% and review site Yelp Inc YELP.N at 1.8%, said eMarketer. Verizon Communications Inc VZ.N owns Verizon Media.