LONDON (AP) — Britain's domestic spy agency MI5 is tracking a number of terrorism plots against the U.K., despite successes in curbing the threat from extremists since deadly attacks on London's transit network five years ago, an official said Wednesday.

Prime Minister David Cameron marked the fifth anniversary of the July 7, 2005, suicide bombings on three subway cars and a bus, sending a wreath and handwritten note to be laid at a memorial to the 52 commuters killed in the attack.

"It was a dreadful day, but it is also a day that will remain, I believe, a symbol of the enduring bravery of the British people," Cameron told the House of Commons.

As the bereaved and injured marked the anniversary, security officers said they continue to monitor hundreds of suspected extremists and plots, but believe work at home and overseas has hampered the ability of terrorists to mount major attacks.

Britain raised its threat level to "severe" in January, the second highest level on a five-point scale, in the wake of the failed attempt to bring down a Detroit-bound airliner on Christmas Day.

Security officials said a range of threats at various stages of development are currently being monitored in Britain — but did not offer a specific figure on how many are under surveillance.

"The threat level is at severe, which means an attack is highly likely," said a government security official, who demanded anonymity to discuss such issues. "There are a range of operations being conducted at any one time."

In January, MI5 director Jonathan Evans told lawmakers his officers are monitoring "a couple of hundred cases of one sort or another," and previously confirmed about 2,000 people in Britain are suspected of involvement in terrorism. He has acknowledged that a series of hefty jail terms handed to convicted terrorists in recent years and the killing of senior al-Qaida operatives in Pakistan and elsewhere have led to a fall in the number of major plots in Britain.

Evans was quoted in an agency history published last year as saying recent convictions have had "a chilling effect on the enthusiasm of the plotters."

Abdulla Ahmed Ali, the British ringleader of a plot to bring down trans-Atlantic airliner bound for North America with liquid explosives, was jailed last year for a minimum of 40 years. Briton Dhiren Barot was jailed for life in 2006 over plans to bomb U.S. financial landmarks and luxury London hotels.

Officials also say they now have a better understanding of the links between suspected terrorists, and are better able to comb through the contacts of those arrested to uncover other plotters.

Lawmakers reported last year that police and security officials likely missed possible chances to identify the London bombers as a potential threat ahead of the 2005 attacks.

One of the London bombers was tracked by intelligence officers as early as 2001, and he and an eventual accomplice were monitored in 2004, but dismissed as petty con artists while officials focused on more pressing threats.

In April, MI5 and police claimed to have disrupted an al-Qaida bomb plot against Britain after making a series of arrests across northern England. All 12 suspects detained were later released without charge, though then-Prime Minister Gordon Brown insisted a "major terrorist plot" had been averted.

Cameron told lawmakers Tuesday that "every day, intelligence officers track terrorist threats and disrupt plots."

Tony Blair, prime minister at the time of the London bombings, said in a statement Wednesday that while the four suicide bombers had succeeded in striking London, "in truth, the terrorists failed."

"Britain resolved, even more surely, to defeat the extremism and intolerance which drove the attackers and stand up for the values of freedom, democracy and the rule of law which as a nation we cherish," Blair said.

Families of some of the victims of the 2005 attacks said Wednesday they were saddened by a decision not to hold an official remembrance ceremony.

Graham Foulkes, whose 22-year-old son David was killed in the Edgware Road bombing, said the fifth anniversary was significant and the absence of the prime minister or London Mayor Boris Johnson at the ceremony was disappointing.

Britain's government said other relatives had asked the government to step back this year to allow a more private ceremony to take place.