The phrase "reason is unknown" is correct and is commonly used in written English.
For example, "The reason for the power outage is unknown at this time."
The reason is unknown, but many scientists believe there is a trigger, perhaps a virus, that starts the process.
The precise reason is unknown, but no one believes that his resignation was voluntary, and everyone believes it is connected with AIG's continuing regulatory troubles.
The reason is unknown, but she thought she might have touched guano-soiled rocks and then covered her nose and mouth with her hands because the cave smelled so bad.
At the same strength level, the Ti 8V 1.5Mo 2Fe 3Al alloy has higher toughness and higher ductility than the Ti-1023 alloy, the reason is unknown and should be studied further.
It's not clear how poor sleep may be a possible risk factor for afib, but the researchers noted that other studies have shown that sleep apnea — a disorder in which your breathing repeatedly stops and restarts — is also associated with a higher risk of afib, although the exact reason is unknown.
But because the drugs are less effective, users do not gain the benefits they would by using other types of antihypertensive drugs, and the reason is unknown, said the scientific team that was led by Dr. Curt D. Furberg of the Wake Forest University School of Medicine in Winston-Salem, N.C.
The origin and reason are unknown.
The exact reason was unknown.
Furthermore, one patient did not receive the intervention due to logistical problems and for one person the reason was unknown.
In the remaining three cases, extensive local disease was the reason not to perform surgery in 2 cases and in 1 case the reason was unknown.
Reasons for not undergoing colonoscopy were refusal by the individual (43.1%) or undergoing another FOBT instead of colonoscopy (25.5%); the reason was unknown for 31.4%.
