Okay kiddo, so in 2004 there was a really bad thing that happened in Madrid, Spain. Some people put bombs on trains and hurt a lot of people. It was a very sad time for many families.
Now, after that happened, some people said that they knew who did it. They said that the people who did it were from a group called Al-Qaeda, which is a group of bad people who don't like other people who don't agree with them.
But then, other people said that maybe it wasn't just Al-Qaeda who did it. They said that maybe some other people were involved too, like the government or some other group.
So, there was a big argument and investigation to try and find out who really did it. Lots of people had different opinions and it was really hard to figure out the truth.
In the end, some people were arrested and put in jail for the bombings, but some people still aren't sure if they really did it or not. It's a really complicated and sad situation, and it's important for us to remember how dangerous and hurtful violence can be.