《新概念英语》（第三册）第一课《A Puma at Large》

《新概念英语》（第三册）第一课《A Puma at Large》
Pumas are large,cat-like animals which are found in America.When reports came into London Zoo that a wild puma had been spotted forty-five miles south of London,they were not taken seriously.However,as the evidence began to accumulate,experts from the Zoo felt obliged to investigate,for the descriptions given by people who claimed to have seen the puma were extraordinarily similar.
其中,这下面这句话的句型改如何解释的?
However,as the evidence began to accumulate,experts from the Zoo felt obliged to investigate,for the descriptions given by people who claimed to have seen the puma were extraordinarily similar.