Experts are looking at all the clues they have to find out which group was behind the train bombings in Madrid, which killed 200 and hurt 1,500.
They are questioning five men, but there is still a lot of uncertainty about who did it.
So could Eta or al-Qaeda have done this?
It is not clear yet, but either group could have. Eta insists it was not to blame.
Terrorism experts are exploring whether people from the Spanish terrorist group Eta and Osama bin Laden's terrorist network al-Qaeda worked together.
But al-Qaeda do not usually work with non-Muslim groups.
What is Eta?
Eta is a terrorist organisation which was started in the 1960s.
Over the last 40 years, it has killed or kidnapped hundreds of people and set off lots of bombs to try to get what it wants.
It wants independence for an area of north-eastern Spain and western France called the Basque homeland.
What clues point to Eta?
Some thought Eta may have planted these bombs in Madrid because it wanted to disrupt the general election.
But Eta has officially denied it, and other experts reckon it's not like other bombs Eta has planted. No warnings were given and the blasts killed many more people than they have before.
Experts are looking at all the clues, including traces of bomb material that Eta has used before.
What clues point to al-Qaeda?
A videotape has been found which claims to be from al-Qaeda.
Al-Qaeda is a large terrorist group which has extreme Muslim beliefs and believe terrorist attacks will make Western countries treat Muslims differently in areas like the Middle East.
A man speaking in Arabic said he was the group's European military chief. He claimed al-Qaeda was behind the attacks, but Spanish authorities are keeping an open-mind and are investigating whether it is real.
Some experts believe the bombs are more like those planted by al-Qaeda because they were very well planned and set to kill the maximum amount of people.
Why would al-Qaeda hurt Spain?
Spain is an ally of the United States and supported Britain and America in the run-up to the Iraq war, which al-Qaeda was against.
Soon after the bombs, a London-based Arabic newspaper said they had been sent an e-mail from al-Qaeda claiming it did it because of Spain's support for the war.
The video message said this too.
Could this happen in the UK?
Security experts say it could happen in any country which supported the war in Iraq.
But a lot is being done to make sure people are kept safe and people should not be worried.
A poster campaign in London is making sure people on trains and tubes are aware of what to do if they see any abandoned bags.