Mr Eliot Spitzer, the New York state attorney-general, has subpoenaed documents from AT&T, the telecoms firm, as part of his investigation into conflicts of interest in Wall Street stock research.

Mr Spitzer has been focusing his inquiry on Salomon Smith Barney and its former telecoms analyst Mr Jack Grubman. But the AT&T subpoena raises questions about whether Mr Spitzer may explore the role of Mr Sandy Weill, chairman and chief executive of Citigroup, Salomon's parent, in stock research.

Mr Spitzer's request appears to be focusing on how AT&T selected underwriters when it issued a tracking stock for AT&T Wireless, its wireless division, in early 2000.

Mr Grubman had been well known for holding bearish views on AT&T, his former employer. But he changed his mind about the company shortly before tracking stock was issued. Salomon wound up as one of the lead underwriters of the deal, one of the most lucrative on Wall Street.

Mr Weill's potential role is of interest because he serves on AT&T's board. Mr Michael Armstrong, the telecoms company's chief executive, is a Citigroup director.

The apparent expansion of Mr Spitzer's inquiry comes at a sensitive time in his dealings with Citigroup. Mr Spitzer was understood to be close to wrapping up his inquiry and there were indications Citigroup was undertaking various internal reforms to satisfy him and other regulators.

Mr Spitzer's interest in the AT&T matter weighed on Citigroup stock, which was down $1.14, or 3.24 per cent, at $34.04 (€35.02) in midday trading.

Citigroup said the company was co-operating with Mr Spitzer and denied Mr Weill had ever caused Mr Grubman to change his opinions.

"Mr Weill never told any analyst what to write and any suggestion he did is outrageous and untrue," the company said.

Mr Grubman left Salomon last week under a separation agreement worth $32 million.