French hooded police officers guard the area with other police officers after a knife attack Thursday, Aug. 23, 2018 in Trappes, west of Paris. A man flagged by French authorities as a suspected radical killed his mother and sister and seriously injured another woman in a knife attack Thursday that was quickly claimed by the Islamic State group. AP Photo/Michel Euler

French hooded police officers guard the area after a knife attack Thursday, Aug. 23, 2018 in Trappes, west of Paris. A man with severe psychiatric problems killed his mother and sister and seriously injured another woman in a knife attack Thursday in a town near Paris, officials said. French police shot and killed the man soon afterward. The Islamic State group, which has a history of opportunistic claims, swiftly claimed responsibility for the attack. AP Photo/Michel Euler

French hooded police officers cordon off the area after a knife attack Thursday, Aug. 23, 2018 in Trappes, west of Paris. However, officials were not ruling out a terrorism link in the early stage of the investigation. The prosecutor's office in Versailles, handling the case, said in a statement Thursday evening that motive for the killings "remains uncertain." AP Photo/Michel Euler

Hooded police officers watch out while French Interior Minister Gerard Collomb answers reporters after a knife attack Thursday, Aug. 23, 2018 in Trappes, west of Paris. A man flagged by French authorities as a suspected radical killed his mother and sister and seriously injured another woman in a knife attack Thursday that was quickly claimed by the Islamic State group. AP Photo/Michel Euler

French Interior Minister Gerard Collomb, center, answers reporters after a knife attack Thursday, Aug. 23, 2018 in Trappes, west of Paris. A man flagged by French authorities as a suspected radical killed his mother and sister and seriously injured another woman in a knife attack Thursday that was quickly claimed by the Islamic State group. AP Photo/Michel Euler