NEW YORK (CNN/Money) - The terrorist attacks of two years ago underscored the importance of high tech in the world of security.

Several publicly traded tech companies have become favorites of investors since 9/11/01 as government agencies and businesses stepped up their spending on security.

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Shares of Invision Technologies (INVN: Research, Estimates), which makes explosive detection equipment used in airports, are up more than 760 percent since Sept. 10, 2001. Sales surged 490 percent in 2002 and earnings jumped from 13 cents a share in 2001 to $4.40 last year. But the stock has cooled in recent months as earnings and sales are expected to decline this year.

The only other company that has been approved to make bomb detection devices for the aviation industry is the more diversified defense firm L-3 Communications (LLL: Research, Estimates). Shares of L-3 are up 47 percent during the past two years.

Shares of Invision Technologies have cooled recently but are still among the biggest gainers of the past two years.

Another tech company that has been busy in the past two years is Flir Systems (FLIR: Research, Estimates), which develops thermal imaging devices that can be used for surveillance purposes. The stock has gained 88 percent in the past two years. Revenue increased 22 percent last year and is expected to rise 17 percent this year. Flir's earnings, however, are only expected to increase 3 percent this year after a 44 percent jump in 2002.

SunGard Data Systems (SDS: Research, Estimates), whose disaster recovery services, helped protect and recover data for scores of corporate clients that were housed in the World Trade Center, has also seen a pickup in business. Earnings increased 19 percent in 2002 as sales soared 34 percent. This year, earnings and revenue are expected to increase by about 10 percent. The stock is up 20 percent in the past two years.

But one tech company that received a lot of attention immediately following the attacks has fizzled since then. Identix (IDNX: Research, Estimates), a biometric company that develops fingerprint and facial recognition technology, saw revenue fall 10 percent in 2002. The company lost money last year and is expected to do so again in 2003 and 2004.

Although shares of Identix are still up nearly 50 percent from two years ago, the stock is trading 60 percent below its peak price achieved in December 2001.