Three explosions hit German football team Borussia Dortmund's bus last Tuesday (April 11) ahead of a Champions League home game, injuring a player and a policeman.
Three separate claims of responsiblity have been made so far but no new suspect has emerged.
Here is what we know and the latest developments in the investigation:
At around 7:15 pm (1715 GMT) on Tuesday, three explosions detonated just minutes after Dortmund's team bus left the squad's hotel heading for their quarter-final, first-leg match against Monaco at home.
The blast shattered the bus windows, and Spanish international Marc Bartra, 26, underwent surgery on a broken wrist after he was hit by flying glass.
A policeman on a motorcycle escorting the team bus suffered trauma from the noise of the blasts.
The explosive devices containing metal pieces appear to have been hidden in a hedge and were set off as the bus passed.
The blast had a radius of more than 100 metres (yards), federal prosecutors said, adding that it was lucky the toll was not heavier.
A piece of metal was found bored into the headrest of a bus seat.
The match was postponed for a day.
The probe has been taken over by federal prosecutors, whose remit includes terror investigations.
Since the attack, three claims of responsibility have emerged.
The first claim, in three identical letters found at the scene, suggested an Islamist link.
It referred to the Berlin Christmas market attack in December claimed by the Islamic State group that killed 12 people, as well as Germany's deployment of Tornado reconnaissance missions as part of an international anti-IS coalition.
But security sources have since questioned the authenticity of the letters and experts have suggested that the claim may have been made to send investigators on the wrong trail.
A second claim, purporting to be from the "anti-fascist" far left, was made online, but prosecutors quickly cast doubt on it.
The Tagesspiegel newspaper, meanwhile, said it had received a third claim by email, this time apparently from far-right circles.
The message rails against multiculturalism, saying it was the motive behind the blasts and threatening another attack.
Frauke Koehler, federal prosecutor spokeswoman, said that authorities were examining the email, and that her office was unable as yet to give an assessment of the note.
Investigators said they were examining all leads, with about 100 officers involved in the probe.
"The investigations are continuing in all directions and are running at full-speed," a spokeswoman from the federal crime bureau BKA said.
No suspect has emerged after the sole man in custody -- an Iraqi -- was cleared of involvement.
Nevertheless there are clues to be looked into.
Experts are examining the traces of the bombs, including their composition and how they were detonated.
Forensic investigators have also combed the site for evidence, for instance left behind in the hedges when the bombs were planted.
As the blasts happened close to the hotel where the team was staying, hotel employees and guests have been questioned.
Residents in the area have also been asked for any possible sightings of suspicious individuals, who could have detonated the bombs by remote control.