British police probing an alleged terror plot to blow up U.S.-bound airliners have gathered "substantial material" in their investigation, Home Secretary John Reid said Sunday.

"The police and the security authorities are content that their investigation is rewarding substantial material which would allow them to take forward the judicial process," Reid said in an interview with ABC-TV's "This Week," refusing to reveal details of the evidence.

"The police and the authorities are convinced that there was an alleged plot here. They have intervened. And in the course of the next few days, we'll wait and see what happens in terms of charges."

Investigators have until Monday to question two suspects, and until Wednesday to interview 21 others. Officers may seek to hold the suspects for a maximum of 28 days before charging them. To keep suspects they still wish to question in custody, police must ask a judge to extend the current deadlines.

Several hundred officers involved in the inquiry — one of the largest ever carried out by British police — including dozens of investigators from Britain's 41 police forces based outside London. Britain's domestic spy agency, MI5, is also continuing investigations, but has refused to disclose the nature of its inquiries.

The alleged plot was revealed Aug. 10 by Reid, who said that police and security services had foiled a plan to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."

The announcement triggered stricter security regulations at airports, which included a ban on passengers carrying hand luggage and any liquids onboard planes. It was revealed by U.S. officials that authorities believed the alleged terrorists planned to detonate liquid explosives aboard the aircraft.

When asked if that was possible, Reid told the television program that it was "relatively simple to make quite an effective bomb" aboard an airplane.

"It is a constant search by the terrorists to find ways around our restrictions, around our surveillance, around our security means, around our airports and other transportation restrictions, in order to try and defeat our counterterrorism and to inflict the sort of damage that we've seen before," Reid said.

The new security measures threw Britain's airports into chaos in the days after the alleged plot was foiled, but officials at London's Heathrow airport — Europe's busiest, and the worst affected hub — said on Sunday that while it was busy, things are returning to normal.