Demand for Manhattan office space “lost some momentum” in May, according to Cushman & Wakefield’s latest office market report, which noted that monthly leasing activity in the borough fell below 2 million square feet for the first time this year. There were 1.8 million square feet of new Manhattan office leases signed last month, the brokerage noted, the lowest level of monthly leasing so far in 2016. Year-over-year, leasing activity is down 13.5 percent.