The alley where more than 150 people lost their lives in the deadly crowd crush in Itaewon. The Hamilton Hotel is located on the right side. (Yonhap)

Police have started an investigation into Hamilton Hotel, which stands on one side of the alley where more than 150 people lost their lives in the deadly crowd crush in Itaewon, it said Wednesday. The hotel is currently believed to have built an illegal extension, which led to the narrowing of the alley.

Under the Building Act, pedestrian passage should be wider than 4 meters for the safety of pedestrians. But, the hotel’s building addition has made the alley narrow to 3.2 meters at one end, creating a bottleneck.

Police said it sent 14 members of the special investigation team to the hotel and the residence of its chief executive to obtain documents regarding hotel operations and licensing.

The special investigation team will try to verify how big of an effect the illegal construction had on the crowd crush.

Seoul Mayor Oh Se-hoon also requested that district offices take strict measures against buildings that have illegally extended their structures, during a meeting with district leaders on Wednesday.

Oh added the Seoul city government will embark on an extensive inspection of ongoing construction in areas where large crowds are expected to gather in line with the year-end holiday season -- such as the neighborhoods around Gangnam Station and Hongik University.

Yongsan-gu District Chief Park Hee-young reportedly did not attend the meeting between Oh and district chiefs.