Greater Lowell firms, large and small, involved in fight for freedom.

When the second plane hit the second tower on Sept. 11, 2001, it was clear that the United States was at war.

As details of the attacks emerged, and plans for combat operations in Afghanistan and Iraq took shape, it soon became equally clear that makers of homeland-security and military products would face new -- and potentially profitable -- challenges.

Greater Lowell companies rose to meet the need for devices that could protect soldiers and civilians from future terrorist threats. Some companies modified or stepped up production on existing products; some created new and innovative gadgets.

Among the first to be deployed was Burlington-based iRobot Corp.'s PackBot, a small scout-type robot that was in development at the time of the attacks. PackBot was quickly dispatched to New York City to search for survivors and help assess the structural integrity of buildings near the World Trade Center.

"It was the first time it... went into the real world," said Joe Dyer, president of iRobot's Government and Industrial Robots division.

The company has built about 500 PackBots. Most are deployed in Iraq and Afghanistan, used to investigate places where soldiers fear to tread, keeping troops out of harm's way.

Wilmington-based Textron Systems has also made significant contributions. According to president Richard Millman, the company was producing just one Armored Security Vehicle every three weeks before 9/11.

"A few years ago ... the Army called us and said, we're taking all the ASVs ever built and bringing them to Iraq," Millman said. "They said, instead of one every three weeks, we want 48 a month."

Textron reached that production goal this June. About 400 ASVs are currently deployed in Iraq, Millman said. The company also increased production of components for smart bombs, and developed new ground sensors for use in Iraq.

"The reward comes when a soldier approaches you at a trade show and says, 'I was in your vehicle, I got hit by nine IEDs, and I wasn't hurt,'" Millman said.

On the home front, companies such as Billerica-based Viisage Technology Inc. offer products designed to keep America secure. (Viisage recently bought rival Identix, changed its name to L-1 Identity Solutions, and moved its headquarters to Connecticut.)

Viisage's iA-thenticate personal-identification devices are now in use in more than 2,500 locations in the U.S., said Doni Fordyce, executive vice president of corporate communications at L-1. The company also makes software designed to make U.S. licenses and passports more secure.

"Hopefully our identities will be more secure, and we will not have people that are able to walk in with false identities that intend to harm our general public," Fordyce said.

Billerica-based American Science & Engineering Inc. and Wilmington-based Ahura provide solutions for airport security. AS&E uses its Z Backscatter X-ray imaging technology in a personnel screening system, SmartCheck, and a new baggage-screening device, Gemini. The Backscatter method can detect organic materials, such as plastic explosives or narcotics, that regular metal detectors cannot, said AS&E vice president of marketing Joe Reiss.

AS&E's Z Backscatter Van, released three years ago, has become the most popular vehicle-inspection system ever sold, according to Reiss. The company has sold more than 200 of the vans at home and overseas -- the Department of Defense is a frequent customer.

Ahura's FirstDefender device received attention recently when a terror plot to bring explosives disguised as common liquids onto airplanes was uncovered. FirstDefender uses lasers to identify chemicals, and could be used to scan liquids in passengers' carry-on baggage. The Transportation Security Administration is currently evaluating the potential use of FirstDefenders, according to Ahura CEO Doug Kahn.

In the communications arena, Lowell-based M/A-Com Inc. rolled out its NetworkFirst solution shortly after 9/11 to improve interoperability between first responders. The well-publicized inability of the New York City police and fire departments to talk to each other on 9/11 brought the interoperability problem "into sharp focus," said John Facella, M/A-Com's director of public safety markets. The company, a division of Tyco International, recently inked a deal to supply New York state with $2 billion of equipment, a contract Reiss said was probably the largest of its kind ever awarded in the U.S.

Huge military and homeland-security contracts have been commonplace since 9/11, said Paul Nisbet, a defense analyst with Newport, R.I.-based JSA Research. Nisbet said defense spending has grown from $300 billion to about $500 billion under President George W. Bush, and homeland-security spending drives that number even higher.

"There was likely to be increased defense spending with ... Bush in the White House, but it certainly was accelerated and pushed forward by 9/11," Nisbet said.

More government cash has allowed the defense industry to "take on tasks they wouldn't have dreamed of five years ago," Nisbet said.

Among the benefactors Nisbet cited is Waltham-based defense contractor Raytheon Co., the area's largest employer. Raytheon, which formed an anti-terrorism task force shortly after 9/11, has seen its revenues grow from $16 billion in 2001 to $21.9 billion in 2005. Its products include the Hawk and Patriot missiles, military aircraft and infrared imaging systems.

Raytheon officials declined comment for this story.

Many company executives said 9/11 had a direct impact on the bottom line.

"Our business clearly has grown more rapidly than if 9/11 hadn't happened," Fordyce said.

Dyer said iRobot pre-9/11 was "an exciting but very small company." Now, he said, "we're producing products, not prototypes" and attributed his division's increases in revenue to an accelerated introduction of robots into the military.

Kahn said Ahura initially was looking to get into the telecommunications market when it was founded in 2002, but switched gears when it became apparent that homeland security would be more lucrative.

Kahn said that while working in that field has helped Ahura succeed and pleased the company's investors, it has also heartened employees.

"I've been involved in many different businesses, and I've never had a greater sense of satisfaction," Kahn said. "I think the entire team is motivated by the ability to develop ... technology that really, truly makes the world a safer place."