"We were deeply saddened by reports of the murder of Rashid Rehman, an attorney and human rights defender in Pakistan," State Department spokesperson Jen Psaki said.
Rashid Rehman, 50, a member of the Human Rights Commission of Pakistan, was shot at on May 7 in Lahore and later died at a hospital.
The senior lawyer had last month received death threats for representing a university professor accused of blasphemy under strict laws against defaming Islam.
"We continue to coordinate and cooperate closely with Pakistani authorities," she added.
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Rehman was the son of late diplomat Ashfaque Ahmed Khan, and nephew of HRCP Secretary-General I A Rehman.
Pakistan's controversial blasphemy law is often misused to settle personal scores and grudges and to persecute minorities, rights groups allege.
Lawyers defending blasphemy accused remain a target of extremists and fundamentalists.