Cite as: 539 U. S. 510 (2003)
Opinion of the Court
tion of counsel's conduct than an accurate description of their deliberations prior to sentencing.
In rejecting petitioner's ineffective assistance claim, the Maryland Court of Appeals appears to have assumed that because counsel had some information with respect to petitioner's background—the information in the PSI and the DSS records—they were in a position to make a tactical choice not to present a mitigation defense. Id., at 611-612, 724 A. 2d, at 17 (citing federal and state precedents finding ineffective assistance in cases in which counsel failed to conduct an investigation of any kind). In assessing the reasonableness of an attorney's investigation, however, a court must consider not only the quantum of evidence already known to counsel, but also whether the known evidence would lead a reasonable attorney to investigate further. Even assuming Schlaich and Nethercott limited the scope of their investigation for strategic reasons, Strickland does not establish that a cursory investigation automatically justifies a tactical decision with respect to sentencing strategy. Rather, a reviewing court must consider the reasonableness of the investigation said to support that strategy. 466 U. S., at 691.
The Maryland Court of Appeals' application of Strickland's governing legal principles was objectively unreasonable. Though the state court acknowledged petitioner's claim that counsel's failure to prepare a social history "did not meet the minimum standards of the profession," the court did not conduct an assessment of whether the decision to cease all investigation upon obtaining the PSI and the DSS records actually demonstrated reasonable professional judgment. Wiggins v. State, 352 Md., at 609, 724 A. 2d, at 16. The state court merely assumed that the investigation was adequate. In light of what the PSI and the DSS records actually revealed, however, counsel chose to abandon their investigation at an unreasonable juncture, making a fully informed decision with respect to sentencing strategy
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