Chinese advertising firm Focus Media Holding's (FMCN.NASDAQ, 8112.HKG, F10.FRA) shares fell to a four-month low on Thursday on fears that US regulators would probe a Carlyle Group buyout of the company, Bloomberg reported. Focus fell 2.8% to US$25.08 by 2:03pm on the New York Stock Exchange, the sharpest fall in one day since September 24. The firm disclosed in a January 18 regulatory filing that the US Securities and Exchange Commission is investigating the buyback and sale of firms that include Allyes Online Media Holding. The company agreed in December to be acquired for US$3.7 billion by a group of investors led by Carlyle Group (CG.NASDAQ). Focus, the target of Muddy Waters allegations in 2011 that said it exaggerated its network, joins other Chinese firms in the process of exiting US stock markets after reporting concerns caused a decline in value.