Former Arsenal and Spain forward Jose Antonio Reyes was driving at more than 220kph when his car crashed, Spanish police have confirmed.
Reyes' funeral took place in his hometown of Utrera on Monday following the fatal accident on Saturday.
The 35-year-old was registered with Segunda Division outfit Extremadura at the time of his death, having signed a short-term deal earlier this year.
A statement from former club Sevilla which announced Reyes' death said his cousin, Jonathan Reyes, also died in the crash on the A-376.
The Guardia Civil confirmed on Tuesday evening that an investigation into the incident was continuing, with speeding the "main cause" if not yet ruled the only factor. They said the former Arsenal man had been the driver of the car.
"The speed limit in the area is 120kph. We can confirm it (the speed of the vehicle) was the main cause of the accident," a Guardia Civil spokesman said.
"Our investigation remains open, we do not know if there are more causes of the accident. But right now we can confirm that the main (cause) was the speeding, it was more than 220kmph."
At Reyes' funeral, his coffin was draped in the flag of Sevilla, where the forward came through the ranks before returning in 2012 to win three Europa League titles under current Arsenal boss Unai Emery.
Emery and former Gunners manager Arsene Wenger were among those who paid their respects, as well as team-mates and other players.
Saturday's Champions League final between Liverpool and Tottenham - hosted by another of Reyes' former clubs, Atletico Madrid - was preceded by a period of reflection following the news earlier in the day.
Capped 21 times by Spain, Reyes left Sevilla for Arsenal in January 2004 and was part of the 'Invincibles' side which went through that season unbeaten to win the Premier League.
He also won the 2005 FA Cup despite being sent off in the final against Manchester United, before a loan spell at Real Madrid saw Reyes lift the LaLiga title.
Two Europa League wins followed after a permanent move to Atleti, and a return to Sevilla in 2012 saw him lift the trophy a further three times.