A design for a pair of residential towers to be built in Yongsan, Seoul, is evoking painful memories for the families of people who were killed in the 9/11 terrorist attacks.

The family members say that a mockup of two apartment buildings connected in the middle by a cloud-like bridge reminds them of the clouds of debris that exploded from the World Trade Center towers.

Foreign newspapers such as the U.S.’ New York Daily News and Britain’s Daily Mail report that bereaved families are issuing complaints about the design.

Netherlands based architecture firm MVRDV said it deeply regrets any unintentional connotations its design evokes regarding the 9/11 terrorist attacks.

MVRDV did not indicate any plans to redesign the buildings. The towers are estimated to cost 20 billion won.