这是个非限制性定语从句,which前一般要加逗号的.which指代的是“17 other vehicles were damaged”整句话的意思
给你翻译一下：17辆车被毁坏了,权威人士说,这（次事件）针对的是美国的车辆.
这里的“appear to ”可以理解为“是,似乎,看来”
target可以作动词,也可以做名词.这里作动词,解释成“以...为目标,以...为目的,针对”.
official say可以看成插入语,你可以把它省掉看,整句话变成：
17 other vehicles were damaged which appear to target US vehicles.
再简化一下的话,把“17 other vehicles were damaged”换成“This”,再把连接词“which”去掉,就成了：
This appear to target US vehicles.这看起来是针对美国车辆的.
说了这么多,应该明白了吧
祝good good study ,day day up~