The Peace Arch Crossing has been closed since the shooting at 2 p.m. Tuesday. The estimated time for reopening is 4 p.m. Wednesday, according to the website for the British Columbia travel information agency DriveBC. Drivers have been diverted to other crossings.
After wounding the guard, a man in a van with Washington license plates shot and killed himself, the RCMP said.
The guard was identified as Lori Bowcock.
The name of the gunman has not been released, and the motive for the shooting is unknown.
Investigators have been interviewing witnesses and reviewing video. The Integrated Homicide Investigation Team has taken over the case as an attempted murder, RCMP Sgt. Jennifer Pound told The Canadian Press.
The Canada Border Services Agency officer was shot in the neck, said RCMP Cpl. Bert Paquet.
Police were confident the man took his own life, Paquet said Tuesday evening.
The Peace Arch crossing in Blaine is the third-busiest port of entry on the northern border. Last month, it averaged 9,000 U.S.-bound cars a day, according to U.S. Customs and Border Protection.
The crossing features a park with a 67-foot-high monument in the form of an arch that connects the U.S. and Canada.
Canadian Brian White told reporters at the scene he was waiting to cross northward when he heard a shot. Guards immediately responded and officials questioned everyone waiting to cross, he said.
Kevin McAllister, assistant general manager at the Peace Portal Golf Course, which is adjacent to the border crossing, said an employee and several guests told him they heard shots fired at around 2 p.m. Tuesday.
"Two shots were fired," he said. "We've heard fire, police, ambulance heading southbound on (Highway) 99, which is probably about a couple hundred yards from the 18th green. So that's what they heard when all hell broke loose."
McAllister said he also heard and saw a police helicopter hovering over the 10th and 11th fairways, which are the closest fairways to the highway and the border crossing.
Paquet said Canadian officials were sharing information with U.S. authorities. "This is a very serious incident that occurred right on the border line," he said.
Washington Gov. Chris Gregoire said she spoke to British Columbia Premier Christy Clark and pledged the state's cooperation and help in the investigation.
"This tragedy hits especially close to home, and reminds us all that our public safety officers put their lives on the line every day to protect the rest of us," Gregoire said.
Her remarks were echoed by the president of the Canada Border Services Agency.
"This is a profound reminder of the risks that border services officers assume every day," Luc Portelance said in a statement from Ottawa. "I know that the courage and dedication of our officers are second to none."