The shootings of police officers in Texas and Missouri on Sunday were the latest in what law enforcement officials say is an alarming spike in ambush-style attacks.

One-third of police officers shot to death on the job this year were purposely targeted by their assailant, according to the National Law Enforcement Officers Memorial Fund.

The San Antonio detective was writing a traffic ticket in his squad car on Sunday morning outside police headquarters when he was shot to death. A St Louis police sergeant who was shot twice in the face on Sunday evening while he sat in traffic in a marked police vehicle is expected to survive.