U.S. regulators have stepped up and expanded their investigation into Taser International Inc. (TASR), the stun-gun maker said Tuesday, sending its shares to a 22-month low.

The move by the Securities and Exchange Commission (search) puts further pressure on Taser, which faces criticism of the safety of its weapons and a range of lawsuits from shareholders and families of Taser victims.

Taser's shares plunged 96 cents, or 13 percent on Nasdaq, closing at $6.35, their lowest close, adjusted for stock splits, since late November 2003. The stock has fallen 80 percent so far this year.

According to Taser, the SEC has escalated its probe to a formal investigation, allowing it to subpoena documents. The SEC had opened an informal probe of the company over statements about the safety of its stun guns and the accounting for a distribution deal struck late in December 2004.

The company also said the SEC is looking into the possible unauthorized acquisition of private information by individuals outside the company in an effort to manipulate Taser stock.

Taser Chief Executive Rick Smith said in a statement that he hoped the SEC probe would address "all pertinent issues," including what he said was data indicating that "there may be a large, improper, naked short position" in the stock.

Short sellers profit when a stock falls by selling borrowed shares of a company, and buying the shares back later at a lower price for return to the lender -- and pocketing the price difference.

"Naked" short selling happens when a person sells short a stock he hasn't borrowed, which can result in what's known as a "fail to deliver" after the settlement date.

The escalation of the SEC's investigation comes as Taser faces continued criticism of its weapons, which disable victims with a 50,000 volt shock. Taser has faced a number of wrongful death suits from families of victims this year, though none has been successful.

Human rights group Amnesty International (search) has logged more than 100 Taser-related deaths since 2001. Taser says its "non-lethal" weapons actually save lives when used by police officers as an alternative to guns.

Taser's stock took off in September 2003, as more and more U.S. law enforcement agencies adopted the company's stun guns, zooming from $2.22, split adjusted, to more than $30 by the following April. After a period of volatile trading, it hit an all-time high of $33.45 on Dec. 30, 2004. The stock closed 86 percent lower than that Tuesday.