A leading Spanish senator and ex-mayor of Valencia, Rita Barbera, has died of a heart attack at 68, doctors say.
On Monday, she testified before the Supreme Court over claims of money-laundering by the ruling Popular Party (PP) during her 24 years as mayor.
Paramedics tried to resuscitate her for half an hour after receiving an emergency call from a Madrid hotel at about 07:00 local time (06:00 GMT).
The Supreme Court began investigating Ms Barbera in September.
Many of her team have been accused of corruption, but Ms Barbera denied any involvement in a suspected illegal financing network.
A lower court judge initially requested an investigation in April, but Ms Barbera's status as a senator, granting her immunity from the lower courts, meant she could only be investigated by the Supreme Court.
The investigation relates to Operation Taula, which saw 24 people arrested in January and is focused on the activities of local officials in Valencia, Alicante and Castellon.
All those held have links to the PP, the dominant political force in the region until elections brought a move to the left in May 2015, and the PP lost its absolute majority in Valencia.
Prime Minister Mariano Rajoy paid tribute to Ms Barbera, who he said "gave everything for Valencia and the Popular Party".
Ms Barbera enjoyed a long period as mayor of Valencia - winning an absolute majority in five consecutive elections between 1995 and 2011.
Under her tenure, she vowed to create a city that rivalled Barcelona. The stunning City of Arts and Sciences cultural and architectural complex and the Norman Foster-designed Congress Centre were both built during the 1990s.
But in 2015 she was voted out, and took up a seat in the Senate.
In January 2016, Spanish authorities made a string of arrests under Operation Taula, amid evidence that a string of PP officials in Valencia had participated in laundering money that funded past election campaigns.
Although Ms Barbera did admit making one donation of €1,000 (£860; $1,060), she denied receiving anything in return.
But while political leaders expressed their condolences, reaction to the former mayor's death was mixed. The left-wing Podemos party filed out of parliament when a minute's silence was held in the chamber.
"We regret the death of Barbera but cannot participate in a political tribute to someone whose career is marked by corruption," tweeted Podemos leader Pablo Iglesias.