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Apple shares tumbled shortly after the start of trading on Monday, briefly suffering their largest price drop in at least three months on an unusual spike in volume.

Large sell orders were seen at 9:51 am ET with more than 6.7 million shares trading in a one-minute stretch, the heaviest minute of trading in Apple since Oct. 29.

The stock lost over three percent in that minute, falling as much as 6.4 percent to $111.27. At midday, it was down three percent to $115.40.

The cause of the decline was not yet clear, though traders pointed to high-speed algorithmic trading programs as a potential culprit. Steve Hammer, a trading educator and founder of HFT Alert in Santa Barbara, Calif., which monitors algorithmic trading, said about 300 different stocks showed elevated price traffic beginning about 9:50 am ET, a sign of institutions putting on sell programs.

“When you see that kind of price action that is simply algos running stocks,” he said.

A sharp price move coupled with high volume often prompts speculation about the influence of high frequency trading (HFT), when computer algorithms are used to trade stocks at an extremely rapid pace. HFT has been criticized for affecting the trading of stocks by sending in numerous trade quotes that slow trading — without filling the trades when shares fall.

“What that is called is evaporation of liquidity, liquidity that was never there in the first place and it’s a typical maneuver that goes on in the fragmented stock market we have now,” said Joseph Saluzzi, co-manager of trading at Themis Trading in Chatham, New Jersey.

Similar declines on heavy volume, though not as dramatic, were seen in other stocks, including Alibaba Group Holding Ltd, which fell 1.4 percent in one minute, and the S&P 500 tracking ETF, which had its busiest minute of trading on Monday at 9:51 am, when nearly 1.5 million shares traded.

At the day’s low, Apple lost more than $40 billion in market value.

Morgan Stanley strategists dropped Apple’s weighting in their strategic portfolio to three percent from four percent in an equity outlook note released Monday, but traders said the swiftness of the decline was too dramatic to be attributed solely to the note, which was released before trading opened.

Saluzzi said “maybe it was the Morgan Stanley news that kind of stimulated the event,” but not enough to cause such a decline.

(Additional reporting by Ryan Vlastelica and David Gaffen; Editing by Jeffrey Benkoe)