Given the following context:

In September 2010, Mikael Åkerfeldt stated that he was writing for a new Opeth album. The band announced on their website that they would start recording their tenth album on January 31, 2011, at the Atlantis/Metronome studios in Stockholm, once again with Jens Bogren (engineering) and Steven Wilson from Porcupine Tree as co-producer.Shortly after mixing was complete on the new album in April 2011, Opeth announced that Per Wiberg was relieved of his duties in the band.
In the press statement, Mikael Åkerfeldt explained the decision, saying, "Mendez, Axe and Fredrik and I came to the decision that we should find a replacement for Per right after the recordings of the new album, and this came as no surprise to Per. He had, in turn, been thinking about leaving, so you could say it was a mutual decision. There's no bad blood, just a relationship that came to an end, and that's that."Opeth's tenth album, Heritage, was released on September 14, 2011, to generally favorable reviews. The album sold 19,000 copies in the United States in its first week of release and debuted at number 19 on the Billboard 200 chart. Heritage debuted at number four in the band's native country of Sweden.Heritage became the second Opeth album to not feature any death growls and had a much more progressive style than previous albums from the band, something that Åkerfeldt had been wanting to do for some time.The first two songs Åkerfeldt wrote for Heritage were in the style of Watershed. After hearing the songs for the first time, Martín Méndez told Åkerfeldt that he would be disappointed if the album continued in that direction. Relieved that Méndez was not interested in doing another conventional Opeth album, Åkerfeldt scrapped the two songs and started the writing process over in a different style. In the press release for Heritage, Mikael Åkerfeldt revealed that he felt as though he had been building to write the album since he was 19 years old. In a review for Allmusic, Thom Jurek called Heritage the band's most adventurous album,...

answer the following question:

What is the last name of the person who had been thinking about leaving the band? 

Wiberg