Lawyers and intelligence experts with direct knowledge of two intercepted terrorist plots that the Obama administration says confirm the value of the NSA's vast data-mining activities have questioned whether the surveillance sweeps played a significant role, if any, in foiling the attacks. The defence of the controversial data collection operations ... has been led by Dianne Feinstein, chairwoman of the Senate intelligence committee, and her equivalent in the House, Mike Rogers. The two politicians have attempted to justify the NSA's use of vast data sweeps such as Prism and Boundless Informant by pointing to the arrests and convictions of would-be New York subway bomber Najibullah Zazi in 2009 and David Headley, who is serving a 35-year prison sentence for his role in the 2008 Mumbai attacks. But court documents lodged in the US and UK, as well as interviews with involved parties, suggest that data-mining through Prism and other NSA programmes played a relatively minor role in the interception of the two plots. Conventional surveillance techniques, in both cases including old-fashioned tip-offs from intelligence services in Britain, appear to have initiated the investigations. The Headley case is a peculiar choice for the administration to highlight as an example of the virtues of data-mining. The fact that the Mumbai attacks occurred, with such devastating effect, in itself suggests that the NSA's secret programmes were limited in their value as he was captured only after the event. Headley ... had been an informant working for the Drug Enforcement Administration perhaps as recently as 2005. There are suggestions that he might have then worked in some capacity for the FBI or CIA.