Google increased its stranglehold on the U.S. search engine market, reaching a record high in May, according to the latest comScore report. Meanwhile, Yahoo’s share of the search engine market further eroded for the ninth straight month. Bing’s share held steady.

Google’s search hit 66.7 percent in May (up from 66.5 in April and up from 65.5 percent from May 2011). Previously, Google’s highest recorded share was 66.6 percent, recorded in December 2010.

We reported on a surge in Google searches following the launch of Google’s Knowledge Graph feature last month. Perhaps this new feature accounts for why Google was able to reach its record high.

Meanwhile, Yahoo continued its slide, reaching a new low of 13.4 percent (down from 13.5 percent and down from 15.9 percent in May 2011). Yahoo, which once upon a time was neck-and-neck with Google, has lost market share since September 2011.

Bing’s search engine market share remained unchanged in May at 15.4 percent. However, Bing was up year-over-year, having inrecreased its share of searches from 14.1 in May 2011.

Ask also remained unchanged at 3 percent in May (but up slightly from its 2.9 percent share in May 2011), while AOL dropped to 1.5 percent (down from 1.6 percent in April, but unchanged year-over-year).

For May, comScore reported Bing-powered searches fell slightly to 25.6 percent, down from 25.9 in April. Google powered 68.9 percent of all organic search results (up from 68.7 percent in April).

More than 17.5 billion “explicit core” searches were conducted in May, up from 17.1 billion in April. Google ranked first with 11.7 billion searches (up from 11.4 billion in April); Bing was 2.7 billion (up from 2.6 billion); Yahoo with 2.3 billion (same as April); Ask with 521 million (up from 511 million); and AOL with 268 million (down from 271 million).