British officials refused to comment Monday on a British Broadcasting Corp. report that al-Qaida sympathizers had tried to infiltrate the MI5 spy agency. The BBC reported that "a number" of infiltrators had been caught trying to join MI5, the country's domestic intelligence agency. It gave no indication of when the alleged incidents might have happened. MI5 is in the midst of a campaign to boost its strength by 50 percent, some 1,000 people, over the next few years. Britain's interior ministry, the Home Office, would not comment directly on the BBC report. "All applicants for jobs at MI5 are subjected to a rigorous vetting procedure and a number of candidates are turned down on security vetting grounds," a spokeswoman told The Associated Press on the government's customary condition of anonymity. "MI5 is very alive to the threat of people who are trying to infiltrate both the security service and police for reasons that would not be good for national security," a government official told the AP, speaking on condition of anonymity because they were not authorized to discuss national security issues.