The luckiest break in the investigation of the Madrid train bombings - finding an unexploded bomb - has delivered spectacular results once again.
Data gathered from one bomb that failed to explode on March 11th - when 10 others killed 191 people on four Madrid commuter trains - had previously led to two dozen arrests.
This time it led police to raid a suburban Madrid apartment on Saturday where four top train bombing suspects blew themselves up rather than give in to capture, killing an elite police agent in the process.
At least two of the suspected Islamist radicals has been the subject of international search warrants issued by the judge investigating the case.
Spanish newspaper El Pais said police swooped on the house in the suburb of Leganes after a suspect activated a mobile phone SIM card, tipping off investigators to his whereabouts.
Police knew about a batch of pre-paid SIM cards that were probably in the possession of the bombers because one of them was recovered from the unexploded bomb.
An Interior Ministry spokesman today declined to confirm the detail about an activated SIM card, saying "we don't want the bad guys to know what we know." But nor did he deny it was true.
Moreover, the unexploded bomb was found, literally, by dumb luck. Without it, investigators would only know a fraction of what they now know with nearly all the suspected culprits dead or in jail.
Ten bombs aboard four packed commuter trains went off almost simultaneously on March 11th, killing 191 people and wounding 1,900 others.
Three other bombs that did not go off were destroyed on the spot in controlled explosions. That is standard procedure by police concerned they could have been detonated by remote control or timer in order to kill police responding to the first set of blasts.
But one more dud - the 14th bomb - went undetected for 12 hours until a mobile phone alarm sounded amid luggage that had been taken from the bombed trains to a police station.
The mobile phone alarm was rigged to trigger the detonator. The bombers set the alarm for roughly 7:40 p.m. instead of 7:40 a.m., when the others exploded on the trains.
Had the detonator functioned, it would have blown up the police station. Instead police traced its pre-paid SIM card to a small mobile phone shop in the multi-ethic Madrid neighbourhood of Lavapies.
From there police arrested the shop's three owners, all of whom have been formally accused of 190 murders and at least 1,400 attempted murders. Evidence gathered at the shop has led to all 24 arrests so far. Nine of those have been set free, leaving 15 in jail.