Forensic tests confirmed the bullet used to shoot a woman in Fredericksburg, Virginia, on Friday came from the same weapon used in four of the six attacks in Washington and its Maryland suburbs.
The 43-year-old woman, who was shot in the back outside a craft store in Fredericksburg, Virginia, some 50 miles (79 km) south of Washington, is now in a stable condition in hospital. She has not been named.
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Local and federal investigators are continuing to track down thousands of leads in pursuit of the sniper now thought responsible for all seven attacks.
Police fear the gunman, who apparently shot his victims at random, may kill again.
"My level of concern about the ability of the suspect to strike again is high," said Charles Moose, police chief in Maryland's Montgomery County, where five of the victims died.
"We implore him to surrender, stop this madness."
But he also urged residents of the US capital and its suburbs to try to continue with their weekend as normal.
No pattern
So far, there appears to be little pattern between the victims - men and women aged from 25 to 72, of white, black, Hispanic and Indian origin.
About 100 local police officers and 50 agents from the federal Bureau of Alcohol, Tobacco and Firearms are now working on the case, Mr Moose said.
Some 600 of 2,500 leads received are believed to be credible, he said.
Police said earlier they were searching for a gunman and the driver of a white van who may have taken him from killing to killing, but have not ruled out there being more than one shooter, or copycat criminals.
A reward of $50,000 has been offered for information leading to an arrest and conviction.
Mr Moose said: "This case remains our highest priority and people certainly still have high anxiety," he said.
Death toll
All six deaths were caused by a single shot which police believe may have been fired from 100 yards or more across car parks.
These who died have been named as:
"
My level of concern about the ability of the suspect to strike again is high
"
Police Chief Charles Moose
Related to this story:
Sixth victim linked to Maryland shootings
(05 Oct 02 | Americas)
US police hunt for Maryland killer
(04 Oct 02 | Americas)
The United States of murder
(19 Aug 98 | In Depth)
Internet links:
Montgomery County Department of Police
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Bureau of Alcohol, Tobacco and Firearms
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