Background: Interpol is an American rock band from New York City. Formed in 1997, the band's original line-up consisted of Paul Banks (vocals, rhythm guitar), Daniel Kessler (lead guitar, vocals), Carlos Dengler (bass guitar, keyboards) and Greg Drudy (drums, percussion). Drudy left the band in 2000 and was replaced by Sam Fogarino. In 2010, shortly after recording finished for the band's fourth album, Dengler left to pursue personal projects.
Context: On March 6, 2009, the band announced on its website that it was working on songs for a fourth album. The album was recorded in Electric Lady Studios during spring 2009. In an interview, Fogarino referred to the album as having gone back to the original sound of Turn On the Bright Lights. It was later claimed by Banks that it would not sound anything like their debut album and that there is some very "classical stuff going on" with it. No news about the album surfaced until late April when the band sent an email directing users to a free download of "Lights", the first officially released song. Citing management changes at the label, the band left Capitol records prior to the album's release, signing again with Matador Records.  The band's fourth album (self-titled) was officially released on September 7, 2010. It was Dengler's last effort with Interpol. In an announcement on the band's website on May 9, 2010, it was revealed that he had left the band sometime after the album's completion. When the remaining members toured in support, he was replaced by multiple players, including David Pajo (formerly of Slint and many other bands) on bass, and Brandon Curtis of The Secret Machines on keyboards and vocals. The album was released on Matador Records in the US, and on Cooperative Music for Europe, Australia and Japan. On February 2011, Pajo announced that he was no longer touring with Interpol, in order to dedicate more time to his family. Pajo was subsequently replaced by Brad Truax.  The band was announced as the opener for the third and fourth legs of U2's 360deg Tour, and subsequently the band announced eighteen US shows to take place over the third leg of U2's 360deg Tour, but only three of them went ahead after Bono's back injury cancelled U2's entire third leg. The band toured the UK and Ireland in November and December 2010.  On June 22, 2010, a promotional video for "Lights", directed by Charlie White and featuring Lola Blanc, was made available for free download on the band's official website.  The band opened for U2 on their rescheduled tour date at Soldier Field in Chicago, Illinois, on July 5, 2011, and remained on U2's 360deg Tour for the remainder of the summer.
Question: What instrument did Carlos Dengler play?

Answer: