washington two u.s. senators thursday urge federal authorities halt plan expansion billion airport facial scan program say technology use identify travelers flight depart nine u.s. airports international destinations may accurate enough raise privacy concern congress approve use program non-u.s. citizens never expressly authorize use americans department homeland security say system need prevent travelers leave country use someone else identity prevent visitors unite state overstay visa senators mike lee republican edward markey democrat letter department homeland security secretary kirstjen nielsen raise concern many travelers would inconvenience faulty scan result question americans subject screen know biometric exit detection technology letter raise objections expand program beyond nine airports already use request dhs stop expansion program provide congress explicit statutory authority use expand biometric exit program u.s. citizens senators write specific authorization request explanation dhs believe authority proceed cite report release thursday georgetown university law school center privacy technology find dhs conduct scan without basic legal technical safeguard meaningful justification billion-dollar cost congress 2016 authorize spend billion 10 years facial scan u.s. customs border protection cbp website say program collect facial image travelers unite state flight use image verify identities say image store two weeks say cbp dedicate protect privacy travelers government say thursday u.s. citizens may opt facial screen instead separate review id document u.s. customs border protection spokeswoman say government work establish biometric exit program way efficient secure traveler least disruptive travel industry airports use system include boston las vegas miami new york john f. kennedy washington dull houston airports chicago hare atlanta senators want dhs provide data program unduly burden travelers dhs say previously goal 96 percent true accept rate mean technology positively identify 96 percent face scan senators however say mean would still false denial one 25 travelers evidence certain face scan exhibit different error rate depend race gender person scan dhs say travelers verify escort another area customs border patrol use methods verify identity georgetown law report also note dhs establish rule govern program dhs hire billion-dollar bouncer check ids never check good spot fake say laura moy deputy director center co-author report also know bias certain group people senators say dhs also need safeguard ensure facial data share u.s. agencies senators seek stop expansion airport facial scan