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CBS 2's Kristyn Harman also contributed to this report.
CHICAGO (CBS) ― Authorities say terrorists planned to use common chemicals to make bombs and blow up 10 flights from Europe to the United States.

CBS 2's Dana Kozlov reports at least one Chicago-bound flight was reportedly on the hit list.

The terrorist attack foiled by British authorities on Thursday was aimed at blowing up as many as 10 airplanes on trans-Atlantic flights, and plotters hoped to stage a dry run within two days, according to U.S. intelligence officials.

The actual attack would have followed within days.

The foiled plot apparently targeted commercial flights on U.S. airlines departing from Britain that were headed toward the most popular American tourist cities. Flights to Chicago, New York City, Washington, D.C., Miami, Los Angeles and Boston were included in the plot, according to a U.S. Justice Dept. source. United, American and Contintenal Airlines were the U.S. carriers picked for the plot.

Officials say the terrorists already had what they needed to carry out the plan when the overseas plot began to unravel.

"I would say this plot was well-advanced. In other words, they had accumulated and assembled the capabilities that they needed and they were in the final stages of planning before execution," Secretary of Homeland Security Michael Chertoff said.

One official said the suicide attackers planned to use a peroxide-based solution that could ignite when sparked by a camera flash or another electronic device.

Officials say Scotland Yard became aware of the group three months ago when two suspects traveled to Pakistan to meet with Islamic extremists. Urgency increased two weeks ago when investigators learned the men were targeting aircraft. Once the suspects picked their flights, investigators moved in.

"The conception, the large number of people involved, the sophisticated design of the devices that were being considered, the sophisticated nature of the plan all suggest that this group was very determined, very skilled and very capable," Chertoff said.

The test run was designed to see whether the plotters would be able to smuggle the needed materials aboard the planes, these officials said. They spoke only on condition of anonymity because of the sensitivity of the subject matter.

British authorities said the attack would have used explosives smuggled in hand luggage. Police described the plot as "mass murder on an unimaginable scale."

Britain said 21 people were arrested overnight, including the alleged "main players" in the plot. The scheme involved 10 flights, one U.S. counter-terrorism official said.

At least 24 people are now in custody, most of them British-born and of Pakistani descent. The youngest suspect is just 17. The police are searching for at least five more suspects, reports CBS News correspondent Sheila MacVicar from London.

As many as 50 people have been involved in the plot and officials describe the terror network as large and global.

CBS News confirmed that one of the arrested men was a Heathrow Airport worker taken from his home by police in his airport uniform. Also, four arrests were made in Pakistan in recent days, two of them "very important," according to officials.

A federal law enforcement official in Washington said that at least one martyrdom tape was found during ongoing raids across England on Thursday. Such a tape, as well as the scheme to strike a range of targets at roughly the same time, is an earmark of al-Qaida.

A senior U.S. counterterrorism official said the suspects, whose ages ranged from 17 to the mid-30s, were looking to sneak at least some chemicals on the planes in sports drink bottles. Teams of at least two or three men were assigned to each flight, the schedules for which they had researched on the Internet, the official said.

Two other U.S. officials said British, American and Pakistani investigators were trying to trace the steps of the suspects in Pakistan and were seeking to determine whether a couple of them attended terrorist training camps there.

American investigators praised Britain for preventing a catastrophe. "If this plot had actually occurred, the world would have stood still," Mark Mershon, assistant director of the FBI, told the AP in New York.

Chicago Airports Affected
The terror alert level at Chicago's airports was elevated to orange -- one step below the highest level -- early Thursday after authorities in London uncovered a terror plot aimed at airlines traveling from Britain to the United States.

The U.S. government banned all liquids and gels from flights, including toothpaste, makeup and suntan lotion. Baby formula and medicines were exempted. Federal officials have said that liquids emerged as a risk from the investigation in Britain. A red level alert, the highest level, has been issued for commercial flights between Britain and the United States.

There is extra security with screenings at checkpoints, and everyone should plan to leave some extra time when flying. Lines were long on Thursday at O'Hare International Airport and Midway International Airport.

The new security measures, among them a ban on liquids in carry-on luggage, took effect at 4 a.m. Thursdya. O'Hare International Airport has its fair share of international flights, with jets scheduled to arrive all day from London-Heathrow. Whether these flights will be arriving as scheduled is yet to be determined.

Passengers are advised to allow extra time and ensure they do not have liquids in their carry-on luggage, Chicago Department of Aviation Commissioner Nuria I. Fernandez said.

"Passengers should plan on arriving at the airports earlier than usual, and at least two hours before their scheduled flight departure," Fernandez said. "They are advised not to bring liquids in carry-on baggage, pack clutter-free, and expect delays at the security checkpoints."

Many moved to lines to check luggage after being handed a one-page memo outlining all the items they could no longer carry on the planes, adding further to the delays.

Changes in security include more canine units, Chicago police officers at every checkpoint, and a heightened local and federal law enforcement presence, she said.

While it is not mandatory to check carry-on luggage, it is preferable to do so, she said.

Breast milk and baby formula are allowed if passengers have children with them. But such substances as hair gel, toothpaste, lotions, and other liquids are forbidden in carry-ons. Earlier, Fernandez said juice may also be carried on for small children, but on Thursday afternoon, she said the rules have been revised and juice is not allowed.

City Chief Emergency Officer Cortez Trotter said Thursday afternoon the Aviation Department received a conference call from the Transportation Security Administration around the time the terror threat was announced, and began planning both short- and long-term strategies to ensure security."

"We are encouraging both patience and vigilance as we all work through this heightened alert together," Trotter said.

Police Supt. Philip Cline said additional officers, as well as bomb-sniffing dogs have been added at airports.

Chicago Transit Authority President Frank Kruesi said there had not been security changes on the CTA because the threat was not directed at rapid transit, but, "We have, in fact, done announcements for all trains that are coming to Midway and to O'Hare" on platforms and within trains to advise of the new restrictions.

The CTA assures riders that every possible security measure is in place for buses and trains, and Amtrak says it's increased its patrols.

Security also remains tight at the Sears Tower, as it has been since the 9/11 attacks.

Public utilities such as Chicago's water filtration plant and Illinois' nuclear power generators also have beefed-up security.

Not too long after the new restrictions were announced, CBS 2 talked with a passenger about the foiled London-based plot to blow up planes mid-flight by planting masked explosives in carry-on luggage.

"There hasn't been a big explosion in the United States since 9/11, so they prevented some stuff, so that's great," said passenger Craig Stockman.

But, Stockman added, "I think we've come a long way with preventing stuff like this but the war in the Iraq – I don't know if that breeds more terrorism or stops it."

Traveler Loekie Paterson said the security changes took effect right away.

"When we got to check in our luggage, they said no liquids, no lipsticks, no toothpastes, nothing, so we had to take everything out of our suitcases and put them on board," Pateron said.

But many air travelers, including Scott Eckert, said they were not concerned about any imminent danger.

"I'm not concerned," Eckert said. "I know that the security here will be very tight."

At O'Hare, there were exceptionally long lines, and large amounts of toiletries and half-consumed bottles of water accumulated in garbage cans and bins set up near security gates. At Midway International Airport, the security checkpoint lines were no longer than usual, but the number of security checkpoints was up to 17 from the usual 14.

As of 8 p.m., activity at the airports was returning to normal Thursday evening

(© 2006 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)