London ? Authorities warned Britons to remain vigilant on Sunday, saying that 24 separate terrorism probes under way showed they could still be in the crosshairs of Islamic militants even after security forces foiled an alleged plot to bring down packed planes heading to the United States.

Tempers flared at the airports, as hundreds of flights were canceled and lengthy security checks caused some passengers to miss their flights.

Home Secretary John Reid said authorities were conducting two dozen separate counterterrorism investigations in Britain, and there was no guarantee the government would be able to thwart every plot.

“We believe we have the main targets from this particular surveillance and plot,” Reid told British Broadcasting Corp. television. “(But) there are still people out there who would carry out such attacks.”

Those sentiments were echoed by U.S. Homeland Security chief Michael Chertoff, who said there was a risk that other groups might try to cause bloodshed on the false assumption that law enforcement and intelligence services might be distracted.

Chertoff also called for taking a renewed look at U.S. laws that could give authorities the flexibility to detain suspects for longer periods of time. Britain recently passed controversial legislation giving the government up to 28 days to hold terror suspects without charge, and the jetliner plot is the first major test of how those new powers will be used.

British police questioned 22 of the suspects in detention Sunday, but authorities remained silent on what, if anything, they have learned.

A line of Passengers snakes through a tent at London's Gatwick Airport North Terminal. On Sunday, flights continued to be delayed and some canceled due to extra security measures imposed after a foiled terror plot to blow up airplanes.

Theories abound

With no briefings by police or government officials, the British press was left to speculate on a wide range of theories.

The Sunday Mirror tabloid asserted that a female suspect in custody may have been planning to use her own baby as a diversion to smuggle a bomb onto the plane, but it did not name its sources. The Sunday Times reported that one of those in custody was believed to be al-Qaida’s leader in Britain, but it did not say which suspect. And The Independent on Sunday cited security sources as saying terrorists were planning an “Apocalyptic wave” of attacks.

There was plenty of time for travelers to soak up all of those theories as they waited in long lines at all the country’s airports, particularly London’s Heathrow and Gatwick.

Almost a third of flights out of Heathrow were canceled Sunday – the airport handles about 1,250 flights a day – and the ban on all carry-on items remained in effect. British Airways canceled almost 100 flights to Europe from Heathrow and scrapped all its domestic flights from Gatwick. Most long-haul flights were operating, although 10 BA flights to the United States were canceled.

Authorities overwhelmed

“It’s disgusting. They don’t have a system,” Jenny Chua, who was waiting Sunday at Heathrow for a flight to Singapore, told a reporter for The Guardian newspaper. “We’ve been trying to call for days and they don’t have enough staff.”

Some airlines have accused the British Airports Authority – which operates seven of the country’s major airports – of being unable to cope with the new anti-terror security requirements. Others appealed to the British government to use police and army reservists to speed up searches at overloaded airport security checkpoints.

“If we the industry and the government don’t work together to have sensible security … we are going to hand these extremists a terrific PR success,” Michael O’Leary, the chief executive of budget airline Ryanair, told Sky News television. “We don’t need to be body-searching young children traveling with their parents on holiday to Spain.”

British Airways acknowledged that some people were missing flights Sunday because they were stuck in security lines.

Eurostar brought in extra staff to deal with the volume of calls from travelers in Britain seeking to go on the high-speed train that stops in France and Brussels.

Police arrested 24 people across England on Thursday, saying they had thwarted a plot to blow up as many as 10 passenger planes flying between Britain and the United States. One suspect was released without charge, and a court will decide today on the detention of another. That last suspect cannot be questioned in the interim.

Another 17 people are in detention in Pakistan – including Rashid Rauf, a British national named by Pakistani intelligence as one of the key suspects. Rauf was picked up along the Pakistan-Afghanistan border, and is believed to have connections to a senior al-Qaida leader in Afghanistan.