AUSTIN, Texas — Police in Texas fear a serial bomber planted four powerful explosive devices that have killed two people and injured four others this month, raising fears in the state capital Austin of another attack.Investigators said they have no clear idea what motivated the series of attacks, which began 17 days ago, and have publicly reached out to the bomber, urging him or her to contact them and explain what his demands are.The first three devices were parcel bombs dropped off at night in front of homes on the city’s east side, with the fourth a trip wire device that went off in a west side neighborhood on Sunday.